Fix Empty String conversion to dot notation

The change from 2.3.0 to 2.3.1 included changing the property compressor checker from a RegExp to a loop (`is_identifier` to `is_identifier_string`). The latter did not account for cases when the string length is zero (i.e. the empty string).

This commit fixes that edge case in `is_identifier_string` and adds a test to the properties test exercising an empty string property name.

Fixes #199