Substring which always gives a result, no matter what offsets are provided.
Substring which always gives a result, no matter what offsets are provided.
Reverse-engineered from MongoDb's $substr op, for lack of a better idea
of how this should work. Note: if start < 0, the result is "".
If length < 0, then result includes the rest of the string. Otherwise
the behavior is as you might expect.