Make saving a just-saved project change nothing

Saving a project that had just been saved changed it again in 18 of the
24 example projects, so a project kept in version control showed changes
nobody made. Both causes were cleanup done on save but not on load:

- Symbol information whose values were all empty was written as an empty
  <elementInformations/> block (DiagramContext::toXml() skips empty
  values, Element::toXml() wrote the block anyway). The next load read it
  as no information and the next save dropped it. The block is now written
  only when something went into it.
- Information values were trimmed on save but not on load, so a label with
  stray spaces (" PRISE") kept them in memory and in its displayed copy
  until the project was opened again. The same rule, kept in one place,
  now applies when reading: stray whitespace around real content trimmed,
  a value that is only whitespace kept (#973).

All 24 examples now save identically a second time (master: 6), and each
one's first save is byte-for-byte what master wrote only on its second.
A title-block property set to a single space keeps it through two saves.

tst_resaveunchanged runs --resave twice on Projet_vierge.qet and
m_000.qet; both fail without this change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/qttest/CMakeLists.txt

@@ -328,3 +328,16 @@ target_include_directories(tst_conductorselfretrace PRIVATE ${QET_DIR}/sources)
 target_link_libraries(tst_conductorselfretrace PRIVATE Qt::Test)
 target_compile_definitions(tst_conductorselfretrace PRIVATE
   "QET_TEST_BINARY_PATH=\"$<TARGET_FILE:qelectrotech>\"")
+
+# Saving a project that was just saved changes nothing: runs the real
+# binary's --resave twice on examples/Projet_vierge.qet (empty information
+# values) and examples/m_000.qet (information values with stray spaces).
+add_executable(
+  tst_resaveunchanged
+  tst_resaveunchanged.cpp)
+add_test(NAME tst_resaveunchanged COMMAND tst_resaveunchanged)
+add_dependencies(tst_resaveunchanged qelectrotech)
+target_link_libraries(tst_resaveunchanged PRIVATE Qt::Test)
+target_compile_definitions(tst_resaveunchanged PRIVATE
+  "QET_TEST_BINARY_PATH=\"$<TARGET_FILE:qelectrotech>\""
+  "QET_EXAMPLES_DIR=\"${QET_DIR}/examples\"")

tests/qttest/tst_resaveunchanged.cpp

@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <QtTest>
+
+#include <QDir>
+#include <QFile>
+#include <QProcess>
+#include <QProcessEnvironment>
+#include <QTemporaryDir>
+
+// Saving a project that was just saved must change nothing. Two things
+// made the second save differ from the first, both cleanup done on save
+// but not on load:
+//  - symbol information whose values were all empty was written as an
+//    empty <elementInformations/> block, which the next load read as no
+//    information and the next save dropped (Projet_vierge.qet);
+//  - information values were trimmed on save but not on load, so a label
+//    with stray spaces kept them in its displayed copy until the project
+//    was opened again (m_000.qet).
+// Runs the real binary's --resave twice on each example.
+class tst_resaveunchanged : public QObject
+{
+	Q_OBJECT
+
+	QTemporaryDir m_dir;
+	int m_run = 0;
+
+	// --resave @p in to a new file, in a sandbox of its own (so a running
+	// QElectroTech cannot answer instead); returns the new file's path.
+	QString resave(const QString &in)
+	{
+		const QString out = m_dir.filePath(QStringLiteral("out%1.qet").arg(m_run));
+		const QString home = m_dir.filePath(QStringLiteral("home%1").arg(m_run++));
+		QDir().mkpath(home);
+		QProcessEnvironment env = QProcessEnvironment::systemEnvironment();
+		env.insert(QStringLiteral("QT_QPA_PLATFORM"), QStringLiteral("offscreen"));
+		env.insert(QStringLiteral("HOME"), home);
+		env.insert(QStringLiteral("XDG_CONFIG_HOME"), home + QStringLiteral("/config"));
+		env.insert(QStringLiteral("XDG_DATA_HOME"), home + QStringLiteral("/data"));
+		QProcess proc;
+		proc.setProcessEnvironment(env);
+		proc.start(QStringLiteral(QET_TEST_BINARY_PATH), {QStringLiteral("--resave"), in, out});
+		if (!proc.waitForFinished(120000) || proc.exitCode() != 0) return {};
+		return out;
+	}
+
+	static QByteArray read(const QString &path)
+	{
+		QFile f(path);
+		return f.open(QIODevice::ReadOnly) ? f.readAll() : QByteArray();
+	}
+
+private slots:
+	void initTestCase()
+	{
+		QVERIFY(m_dir.isValid());
+		QVERIFY(QFile::exists(QStringLiteral(QET_TEST_BINARY_PATH)));
+	}
+
+	void secondSaveChangesNothing_data()
+	{
+		QTest::addColumn<QString>("project");
+		QTest::newRow("empty information values") << QStringLiteral("Projet_vierge.qet");
+		QTest::newRow("information values with stray spaces") << QStringLiteral("m_000.qet");
+	}
+
+	void secondSaveChangesNothing()
+	{
+		QFETCH(QString, project);
+		const QString first = resave(QStringLiteral(QET_EXAMPLES_DIR "/") + project);
+		QVERIFY2(!first.isEmpty(), "first --resave failed");
+		const QString second = resave(first);
+		QVERIFY2(!second.isEmpty(), "second --resave failed");
+		const QByteArray a = read(first), b = read(second);
+		QVERIFY(!a.isEmpty());
+		QVERIFY2(a == b, "the second save changed the file");
+	}
+};
+
+QTEST_APPLESS_MAIN(tst_resaveunchanged)
+
+#include "tst_resaveunchanged.moc"