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.

Use mainCertificate since it handles the logic already.

Test: TestAppMissingCertificateAllowMissingDependencies
Bug: 283102635
Merged-In: : I8b27f65aa7d071041171ad45ac52bf47fa31bf2b
Change-Id: Icaaa45d48ca4edb876687c0813165d107d28ef8a