Fix an issue with empty 'Summary' fields in commit messages

Summary:
If the 'Summary' is not present and not inferred to be empty from a newline after the title with no explicit 'Summary' field, we'll copy all the field values from the revision (including NULL), not overwrite the 'Summary' value from the message (since it's not present) and then write the NULL back to the revision.

Instead, string cast the read from the Revision so we write back empty string in the not-provided case.

Test Plan: Ran "arc diff --create --use-commit-message HEAD" with P336 in HEAD, didn't fail (previously, it failed).

Reviewers: btrahan, 20after4

Reviewed By: 20after4

CC: aran, epriestley

Maniphest Tasks: T1019

Differential Revision: https://secure.phabricator.com/D1956