BUG GridState_Data doesn't hold falsey values

After patch #2522 was taken GridField_Data was no longer able to hold
falsey values. See failing tests.
BUG Fixed handing of false values in GridState_Data
API Added ability to unset values
Merge pull request #2818 from tractorcow/pulls/gridstate_data-error-test

BUG GridState_Data doesn't hold falsey values - Fixes #2813