[fromContentUri] Null/short path causes substring crash

Affected Method (full signature):
com.amaze.filemanager.filesystem.files.FileUtils.fromContentUri(@androidx.annotation.NonNull android.net.Uri uri)

Environment:
• AmazeFileManager: stock release/4.0
• Robolectric/JVM

Steps to Reproduce (Robolectric-friendly):
1. Uri u = Uri.parse("content://com.example.provider/x"); (very short path)
2. Call File f = FileUtils.fromContentUri(u);
3. Method performs substring(FILE_PROVIDER_PREFIX.length() + 1) on uri.getPath() without length check.

Expected Behavior:
Validate length and prefix before substring; gracefully reject/return fallback for malformed content URIs.

Actual Behavior:
StringIndexOutOfBoundsException or NullPointerException when getPath() is null/short.

Minimal Input Example:
android.net.Uri u = android.net.Uri.parse("content://com.example.provider/x");
java.io.File f = FileUtils.fromContentUri(u); // crash or invalid

Impact:
Crashes when browsing or opening content documents with atypical but valid URIs.

Suggested Fix:
Guard uri.getPath() for null/length, verify expected prefix before substring, and handle short paths with a safe fallback (or return null/throw a descriptive exception). Prefer robust parsing over fixed offsets.