pr_monitor: post empty string as commit message if PR body is empty (#5458)
pr_monitor: post empty string as commit message if PR body is empty (#5458)

Fixes #4412

Verified in my personal fork via curling the API directly.

@MichaelBroughton
cc @dabacon 
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq