[image_picker] Handle a null stream from openInputStream

[mboetger]

Null safety when ContentResolver cannot open a stream.

Copied from closed: #12682

Fixes: flutter/flutter#191988

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[gemini-code-assist]

Code Review

This pull request addresses a potential NullPointerException crash in the Android image_picker plugin by adding a null check for the InputStream returned by ContentResolver#openInputStream() in FileUtils.java. If the stream is null, the method returns null. A unit test has been added to verify this behavior, and the package version is bumped to 0.8.13+22. There are no review comments, so there is no feedback to provide.