Only wait for merge delay if there's merge pending am: a4650bd30e am: 66b56141c3

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2886530

Change-Id: I817117aa1bdb146ee18c5142702a0e2da49748ae
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>