R1:0c51885cf7b2

Survive importing Git commits with no commit message and/or no author

Summary:
Ref T13538. See PHI1739. Synthetic Git commits with no author and/or no commit message currently extract `null` and then fail to parse.

Ideally, we would carefully distinguish between `null` and empty string. In practice, that requires significant schema changes (these columns are non-nullable and have indexing requirements) and these cases are degenerate. These commits are challenging to build and can not normally be…