Steps to reproduce
See protractor test:

  Variable
    editing
      when empty
        ✓ has a dropdown 
        ✓ has a name input 
        ✓ has a value input 
        (x) is not valid 

On editing an empty variable with value type set to null, cam-variable-validator directive has no value.

Observed behavior
Validity of input field is true.

Expected behavior
Validity should be false.