Summary:
This PR fixes a bug where clearing default fields did not persist after saving and restarting the app.

Fixes: #1333

Issue:
Users were unable to clear certain default fields, specifically the Sales Invoice Terms. When a field was cleared and saved, the previous value would reappear after restarting the app. (Ref: #1333)

Root Cause:

• When a field was cleared, the value sent to the database was null.
• SQLite and the Frappe Books model treat null as “no value,” which does not overwrite the existing value in the database.
• As a result, the field was repopulated with the previous value on app restart.

Fix:

• Updated the onValueChange method to convert null to an empty string before saving:
  await this.doc?.set(fieldname, value ?? '');
• This ensures that clearing a field sets an explicit empty string in the database, so the cleared value persists.

Impact:

• Fixes clearing behavior for Sales Invoice Terms & Purchase Invoice Terms.
• Also resolves the same issue for any other default fields affected by this null persistence problem.

Testing:

• Manually cleared Sales Invoice Terms & Purchase Invoice Terms, saved, and restarted the app, the field remained empty.
• Verified that saving non-empty values still works correctly.

Demo / Video:

• Clear Sales Invoice Terms field.
• Save settings.
• Restart the app.
• Verify the field remains empty.