Check for null mLeash before setting alpha.

There were few reports in the bug for this to not always be set.
Based on the stack trace it is related to Keyguard showing/hiding,
but I was unable to repro locally the issue.

Adding a safety check to prevent this from occuring with proto log if it happens.

Bug: 233129957
Test: n/a, unable to repro, fix based on the stack trace
Change-Id: Ie45e557601fe52da6003790446b18aad2db681e5