Fix bugtracker #973: unset title-block custom variable shows its own name

BorderTitleBlock::updateDiagramContextForTitleBlock() skips merging a
page-level custom variable into the title block's render context whenever
its value is empty -- added by PR #572 to fix #531, where an empty
page-level value was shadowing a real project-level one of the same name.

But skipping the merge removes the key from the context entirely, and
TitleBlockTemplate::interpreteVariables() only replaces "%name"/"%{name}"
when "name" is an actual key in that context -- anything absent is left as
its own literal placeholder text. Folio Properties auto-adds every one of
a template's custom variables to the Custom tab with an empty value (#271/
#495) precisely so the user only has to fill in what's missing; until they
do, that variable now renders as e.g. "%label1" instead of blank.

Reproduced two ways: a synthetic fixture, and examples/2612_ats_singlephase.qet
itself, which already carries three such auto-added-but-unset properties
("label1", "label2", "label3") and renders all three literally on current
master.

Fix: skip the empty page-level value only when a project-level one already
exists to show through (preserving #531's guarantee); otherwise still merge
it in empty, so the placeholder resolves to blank rather than falling out
of the context altogether.

Verified against the shipped example (--export-png, before/after crop of
the rendered title block): "%label1"/"%label2"/"%label3" now blank. A
variable never added to the Custom tab at all ("%client", also present in
the same example) is unaffected -- nothing was ever configured for it, and
that is a separate, narrower case. Qt 6.10.2, ctest 11/11.

A related but distinct issue -- DiagramContext::toXml() trims a stored
value before saving, so an all-whitespace value is written as empty --
explains a second symptom from the same report (a single-space "workaround"
value vanishing after the project is reopened) but touches every
context-backed property, not just title blocks, and is left for a separate
fix.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/bordertitleblock.cpp

@@ -896,10 +896,20 @@ void BorderTitleBlock::updateDiagramContextForTitleBlock(
 	// An empty page-level value means the variable was auto-added to the
 	// folio's Custom tab (#495) but never actually set by the user, so it
 	// must not shadow a real project-level value of the same name (#531).
+	//
+	// That guard has to stop short of removing the key outright, though
+	// (#973). TitleBlockTemplate::interpreteVariables() only replaces a
+	// "%name"/"%{name}" placeholder when "name" is a key in this context at
+	// all -- an unset variable that never makes it in is left as its own
+	// literal placeholder text in the rendered title block, not blank.
+	// So an empty page-level value is skipped only when a real project-level
+	// one is already there to show through; otherwise it still goes in
+	// empty, which is what makes the placeholder resolve to nothing.
 	DiagramContext context = initial_context;
 	foreach (QString key, additional_fields_.keys()) {
-		if (!additional_fields_[key].toString().isEmpty())
-			context.addValue(key, additional_fields_[key]);
+		const QVariant value = additional_fields_[key];
+		if (!value.toString().isEmpty() || !context.contains(key))
+			context.addValue(key, value);
 	}
 
 	// ... overridden by the historical and/or dynamically generated fields