Bugfix: Allow dirty open handles to deleted files to be closed

[foodprocessor]

Based on code analysis, I think this sequence:

1. Open a file and get a handle
2. Write to the handle
3. Delete the file
4. Close the handle

Would have failed to close the handle in step 4.
The impact on the user application would be a handle leak and maybe other unexpected behavior caused by close failing.

This fix checks if the file was deleted inside flushFileInternal, and skips the upload without returning any errors, allowing close to finish and succeed.

Checklist

• Tested locally
• Added Unit Test
• Updated CHANGELOG.md

[foodprocessor]

Since the fuse driver actually prevents open files from being deleted by secretly renaming files to a hidden name instead of actually deleting them (.fuse_hidden files), this bugfix is not needed. But a fun find anyway.