Fix truthy-ness condition in pipelines that use denoising_start

[a-r-r-o-w]

What does this PR do?

Fixes #6495 and part of #5685

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Who can review?

@yiyixuxu

[HuggingFaceDocBuilderDev]

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[yiyixuxu]

I think we need to update the test now that we fixed that function :)

[a-r-r-o-w]

@yiyixuxu Could you review please? The tests are now passing locally. I think the initial condition of checking isinstance(self.denoising_end, float) every time was incorrect because you can have denoising_end=None but denoising_start as not None.

[yiyixuxu]

thanks!