R1:acd767c7f33d

Allow "differential.createinline" to accept JSON "false" for "isNewFile"

Summary:
See <https://discourse.phabricator-community.org/t/error-creating-inline-comment-via-conduit-api/4535>. See T12678.

This API method currently does not accept a JSON "false", but reasonably should.

Test Plan:
- Called method with "isNewFile: false".
- Before: type error.
- After: inline comment.

Differential Revision: https://secure.phabricator.com/D21522