NIU Actions: Handle null default assistant

This handles the case where Settings.Secure.ASSISTANT is null, which occurs when config_defaultAssistant is not set at build time.

Bug: 195600539
Test: Manual (built locally without config_defaultAssistant)
Test: b/195681795
Change-Id: Ie8da67be5f448847c07438a25b12e8b5b2022f05
1 file changed