Fix an initialization issue in Herald rules in Chrome

Summary:
Fixes T10646. When you load the page or click "New Condition" or "New Action", we try to add a condition and action with some default values.

Currently, the logic just sets everything to null or 'default'. This technically works in Safari, but is less successful in Chrome. (I think Safari prevents you from picking an invalid value.)

Instead of relying on the browser to pick the right value, set the correct value explicitly.

Test Plan:

• Created a new rule in Chrome, Safari.
• Added fields and conditions in Chrome, Safari.
• Edited existing rules in Chrome, Safari.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10646

Differential Revision: https://secure.phabricator.com/D15507