NULL is not a proper value for GStrv yet we cannot escape it in the PDB
since we generate default values for non-passed arguments (especially in
interactive case where most procedure arguments aren't set). And for
such boxed type, it will be NULL.
So when we see a NULL GStrv parameter, let's not ignore it (which will
just crash the plug-in). Simply transform it to a GStrv of size 0.
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