Handled possible NPE in DefaultShortcutCategoriesRepository

API data could potentially send null keyboardShortcutGroup label to
Default ShortcutCategoriesRepository. When this happens we use an empty
string as the subcategory label.

Test: DefaultShortcutCategoriesRepositoryTest
Flag: NONE Trivial
Bug: 415762126
Change-Id: Iad605d6bfd5c9f92284bfee380ec59e3097bd797