[REG 5.8.0 -> 5.9.1] No visual feedback when searching for empty string

1. Start Assistant.
2. Open the "Search" tab.
3. Make sure the line edit for the search phrase is empty.
4. Click "Search".
   Nothing happens.

Instead, Assistant should give any feedback to the user. Assistant 5.8.0 and earlier displayed the message "Your search did not match any documents." where the search results would have appeared.

Found by Squish test tst_HELP04 in Qt Creator's repository.