QgsAuxiliaryLayer::createProperty(*)
Instead, if a property already exists it will be upgraded to
an expression based property of the form:
coalesce("new aux field", 'existing' || 'property' || 'expression')
(i.e. allow per-feature value overrides from the auxiliary field,
but by default fallback to the existing property definition)
Refs #44475
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
