[file_selector_android] Handle a null stream from openInputStream

[mboetger]

Null safety when ContentResolver cannot open a stream.

Copied from closed: #12683

Fixes: #191989

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[gemini-code-assist]

Code Review

This pull request addresses a crash that occurs when the Android content provider returns a null input stream for a selected file. It introduces null checks for the input stream in FileSelectorApiImpl.java and FileUtils.java, returning null or throwing an IOException respectively. Corresponding unit tests have been added to verify these behaviors, and the package version has been bumped to 0.5.2+11. There are no review comments, so I have no feedback to provide.