fix(storage,ios): Handle null Storage metadata values

[bcgilliom]

Summary

It seems that the metadata coming back from firebase is occasionally nil, which throws an exception in NSDictionary, so this probably just hides a deeper problem. But, it's better than crashing!

Checklist

• Supports Android
• Supports iOS
• e2e tests added or updated in packages/**/e2e
• Flow types updated
• Typescript types updated

Test Plan

• use Storage putFile on iOS, should not see a crash related to nil at object[1]
• this was about a 1/5 bug for me, so reproducing might be tricky

Release Plan

[IOS ][BUGFIX ] [Storage] - fix occasional crash when uploading file with putFile

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[codecov]

Codecov Report

Merging #2881 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2881      +/-   ##
==========================================
- Coverage   88.69%   88.68%   -<.01%     
==========================================
  Files         111      111              
  Lines        3437     3436       -1     
==========================================
- Hits         3048     3047       -1     
  Misses        389      389

[Salakar]

Yep, this looks like iOS code to me, approved!

[Salakar]

Thanks for sending this fix over, will release shortly

[Salakar]

This is now live in v6.0.4. Thanks