Fix: Handle Null Byte Array in saveFile Method

[vicajilau]

Context

This PR addresses an issue in the FilePicker.platform.saveFile method where passing a null Uint8List causes failures. This issue is primarily observed on Android (API 35), but the solution is implemented for both Android and iOS to ensure consistency.

Root Cause

The null value for Uint8List was not handled correctly, leading to exceptions during file operations. Specifically:

• On Android, null values caused crashes when attempting to write to the file system.

Solution

The fix ensures that:

1. If a Uint8List is null, it is replaced with an empty Uint8List(0) before being processed.
2. The change has been tested on both Android and iOS platforms and confirmed to function without issues.

Changes

• Updated the saveFile method to replace null Uint8List with Uint8List(0).
• Ensured platform-specific implementations in both Android and iOS handle this case consistently.

Testing

• Verified on Android API 35 to resolve the crash.
• Tested on iOS to confirm that the change does not introduce new issues.

References

• Issue Bug Report: NullPointerException in FilePicker on Android API 35 #1660 in the official repository.
• Discussion and testing logs indicate the problem is limited to null byte arrays.

Notes

This fix aligns with the goal of ensuring robustness in file operations across platforms, avoiding unexpected crashes due to null parameter values.

Checklist

• Code changes reviewed.
• Tested on Android API 35.
• Tested on iOS for compatibility.