[image_picker] Check for failure in iOS metadata updates

[stuartmorgan-g]

Reworks the logic in updateMetaData:toImage: to correctly handle the
possibility of null return values from intermediate steps. Also renames
it to have a more idiomatic ObjC name.

Speculative fix for flutter/flutter#80429
Since the case this would likely fix is bad image data, this may simply
move problems to another part of the plugin, but this is definitely a
correctness improvement here, and if it does move the issue elsewhere we
can tackle that once we have more information.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

I wanted to fix the incorrect capitalization of MetaData, but that's everywhere in this plugin, so that would have been a much bigger change (or inconsistent in this one method, which would be worse than it being wrong everywhere).

[jmagman]

LGTM