The recent change of 19844d27ac fixed this
function for valid values of input/output parameters on input, but broke
it if the parameters had invalid value: in this case, we still need to
fill them even if this attribute doesn't have any specific alignment of
its own.
Account for this case too now, explain the logic of this function in the
comments inside it and extend the unit test to check for this case too.
Now the function should really conform to its documented (and expected,
including by the existing code in various grid renderers) behaviour.
Closes https://github.com/wxWidgets/wxWidgets/pull/1665
811be7ced7