Allow mapping to have an empty string as name

The mapping name check in addMapping was previously for truthiness which meant that an empty string would evaluate to false. This causes an error in the _serializeMappings function when a mapping has an empty string for its name because that check is for != null and thus an entry is expected in _names and it does not exist.
No, @fitzgen explicitly told me that he wanted `!=` since that catches `undefined` as well.