[Python] Fix handling of zero-width types returned by a custom Demangler

[bdash]

Explicitly test the type against None rather than testing for truthiness. This ensures that zero-width types returned by a custom demangler will be applied.

Fixes #8099.

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For some reason Type implements __len__ and returns self.width. This causes zero-width types, such as function types, to be considered False. This is somewhere on the spectrum of unnecessarily confusing and just plain wrong.

[CouleeApps]

... I'm surprised python makes the leap from truthiness -> non zero length. What ever happened to "Explicit is better than implicit."

The bugfix change itself is good but the test you wrote (in the private repo) might be better placed in test_demangler.py and likely doesn't need all the extra scaffolding.

[bdash]

I'll rework my test. I somehow entirely overlooked the existence of that file 😅

Python has treated empty lists as false since very early versions, and they expanded that behavior to user defined types via __len__. I think we could implement __bool__ on Type to avoid this, but I'm not entirely sure why it implements __len__. It's not a sequence or a mapping as Python sees it. len(some_type) evaluating to its width seems kinda surprising, and I'm not sure why anyone would reach for that vs some_type.width. It may be too late to undo that now, though.

[CouleeApps]

I don't think we should make any changes to Type at this time, but it would be worth looking around the codebase to see if anywhere else accidentally (or intentionally?) makes use of this behavior