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!
... and to count them
Tests updated accordingly, note that from now on
the assumption that a layer store or a layer project
contains only valid layers will not be true anymore.
To be honest it has never been true, because a
layer can become invalid at any time during
its life cycle, so better never assume that a
layer from the store or from the project is vaid.
bool(obj) in Python has the following semantics:
1. if the object has __bool__() method, return its value
2. if the object has __len__() method, return its value
3. return True
So for objects in QGIS API that implement __len__() method, we were getting
unexpected behavior - for example, "if layer: ..." would evaluate as False
in case the layer was empty, while the usual expectation is that any reference
to an object that is not None should evaluate to True.
