[device_info_plus_ios] fix identifierForVendor

[alexbatalov]

Description

This PR likely fixes #621. Of all used UIDevice properties only identifierForVendor is marked as nullable. However the circumstances when it is null are hard to reproduce, so it's a guess.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
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Breaking Change

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[miquelbeltran]

LGTM! Like in the other PR, could you also add a new entry on the CHANGELOG and update the version number in the pubspec? thanks!

[miquelbeltran]

hey @alexbatalov some conflicts appeared, could you fix them? please ping me when ready and I will merge

[alexbatalov]

@miquelbeltran I've updated version and changelog, but there is other failing check, see #661, which fixes failing test in package_info_plus, which itself fails because there is a missing changelog in network_info_plus (see #655).

[miquelbeltran]

thanks @alexbatalov I will finish merging and releasing once I am back in the office

[mhadaily]

@alexbatalov can you please resolve the conflicts then I will merge and release a new package.

[mhadaily]

LGTM