Add null check in biometric service

The only way it can be null here if an action caused finished the
session's lifecycle (another error). In this case,
the delayed runnable can be safely ignored.

Fixes: 122462784
Test: Using BiometricPromptDemo, do the following
      1) Fail fingeprint five times
      2) Quickly press negative button
      3) Device does not crash anymore

Change-Id: I67e312da0ec569b64d1cf854ef47ca819490e1cb