Don't panic if no certificates found

Getting the first certificate will panic if there are no certificates,
which can happen when AllowMissingDependencies is set and the
certificate property is a module reference to a missing module.
Only get the first certificate if the list is not nil.

Bug: 228379411
Test: TestAppMissingCertificateAllowMissingDependencies
Change-Id: I046d75dbbd4f21f4a2b6851f558e430e9879fcff