backend/ninja: Fix cases where None is passed when unexpected

When getting debug file arguments we can sometimes pass None, where a
None is unexpected. This becomes a particular problem in the Cuda
compiler, where the output will unconditionally be concatenated with a
static string, resulting in an uncaught exception. This is really easy
to spot once we annotate the functions in question, where a static type
checker like mypy easily spots the issue.

This commit adds those annotations, and then fixes the resulting error.

Fixes: #12997

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