Fix DifferentialGetCommitMessageConduitAPIMethod execute strlen(null)

Summary:
When iterating through the fields of a differential commit, the DifferentialGetCommitMessageConduitAPIMethod execute method explicitly allows a value to be either a string or a null. It then calls strlen upon this possibly null value.

We could replace the strlen with phutil_nonempty_string, but as the code has already eliminated variable types other than string or null, it is more efficient to explicitly check for null or ''
```
$value === null or $value == ''
```

Fixes T15527

Test Plan:
Run
```
arc diff
```

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15527

Differential Revision: https://we.phorge.it/D25332