Handle language being blank (not just nil)

[fbacall]

Summary of changes

• Change Event language validation to accept empty string
• Allow language to be unset when editing an event

Motivation and context

When creating an event through the UI, if you don't select a language from the drop down, the parameter is passed as "", which causes validation to fail.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.