[Approval Needed] Missing Version Metadata When Running Isort from Source code

[maheshsaripalli9]

I'm trying to run Isort directly from the source code and ran into an issue where the module wouldn't work. After some investigation, it seems the problem stems from missing metadata entries in _version.py that would normally be added during a pip install.

I've made update to _version.py so that it defaults to an empty string if the version metadata is not available in #2226.

Could someone please review and approve this change to _version.py?