Fixes feature introduced in #7508 to use the default value if nothing is specified in command line

cc liancheng rxin pwendell

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #7558 from shivaram/merge-script-fix and squashes the following commits:

7092141 [Shivaram Venkataraman] Use default primary author if unspecified