fix: guard startSyncFolderOperation against a null folder (#4738)

[ibrahim-iqbal]

Related Issues

App: closes #4738

ReceiveExternalFilesViewModel.refreshFolderUseCase(folderToSync: OCFile) is a Kotlin function with a non-null parameter. ReceiveExternalFilesActivity.startSyncFolderOperation forwarded whatever came in without a null check, so the two entry points that can hand back a null folder crashed with the Kotlin platform-type NPE reported in issue #4738:

• onSavedCertificate() → startSyncFolderOperation(getCurrentDir()), and FileActivity.getCurrentDir() returns null when getFile() is null, when the current file isn't a folder and the storage manager is missing, or when the storage manager can't resolve the parent path (FileActivity.java:455-466).
• The Play console stack in the issue matches this exact bridge — refreshFolderUseCase (Unknown Source:6) is the Kotlin null check on the parameter.

Changes

• ReceiveExternalFilesActivity.startSyncFolderOperation: short-circuit and log via Timber when folder is null instead of forwarding to Kotlin.

• Kept every other caller unchanged — onBackPressed (line 471), onItemClick (line 500), and the initial spaces-root call (line 259) already null-check the folder they pass in, so no other site needed the guard.

• Changelog file at changelog/unreleased/4738 (Bugfix: type).

• Release Notes in ReleaseNotesViewModel.kt — crash-only, not a user-visible feature; happy to add one if you'd prefer.

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QA

• Local:
  • ./gradlew :owncloudApp:compileOriginalDebugJavaWithJavac → BUILD SUCCESSFUL.
  • No other callers of startSyncFolderOperation had to change; grep confirms four call sites and the three that aren't getCurrentDir() already guard the value.
• Manual repro (attach a file into the app, navigate through folders, dismiss a certificate dialog, then rotate/back) is worth running once during review since the crash is timing-dependent, but the null path is now impossible to reach past the guard.