Return early in SaferIntentUtils.blockNullAction when action is not null

All operations can be skipped if the target intent is not null, making
the most common case: implicit intent query, basically no-op to improve
performance.

Bug: 336039853
Test: atest SaferIntentTest
Flag: EXEMPT bugfix
Change-Id: If162bf2b46bce184213a14200228f1497e76e883