fix: allow undefined value for all fields

[wschurman]

Why

Similar to how null is blanket allowed as a field value, undefined should have similar behavior for "unsetting" a mutator setField call.

For example, some code like:

mutator.setField('key', 3);
if (condition) {
  // unset the field
  mutator.setField('key', undefined);
}

How

Allow undefined in the base class field validation method in the same way null is allowed.

Test Plan

Run tests.