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>

sources/qetgraphicsitem/element.cpp

@@ -1033,7 +1033,11 @@ QDomElement Element::toXml(
 		QDomElement infos =
 				document.createElement(QStringLiteral("elementInformations"));
 		m_data.m_informations.toXml(infos, QStringLiteral("elementInformation"));
-		element.appendChild(infos);
+			//toXml() skips empty values: an element whose information is
+			//all empty would otherwise be written an empty block, which the
+			//next load reads as no information and the next save drops.
+		if (infos.hasChildNodes())
+			element.appendChild(infos);
 	}
 
 		//Save override properties (For now, only used when the element is a terminal)