Only check for HDUs if they are not None (fits reader)

[MSeifert04]

I've added no regression test (because hdu_flags=None is the default and therefore already checked in several tests) and I added no changelog entry because it shouldn't change any behaviour.

The reason this fails is because None in HDUList is not allowed anymore in astropy.io.fits but hdu_mask, hdu_uncertainty and hdu_flags are allowed to be None.

I reported the issue upstream: astropy/astropy#5583 but I think we should change it even if it's not fixed upstream.

[juliantaylor]

this is a bug in astropy, None in container should imo not raise

[embray]

I agree--it's a bug

[MSeifert04]

ok, should be fixed eventually by astropy/astropy#5589.

Thanks for the affirmation @juliantaylor @embray

[mwcraig]

Hmmm, I was inclined to merge this anyway since there is no need to do the search if the key is None.

[MSeifert04]

@mwcraig I restored the branch, if you want to reopen and merge it should be possible.

[mwcraig]

Thanks, merging.