function getInitialValue(props, propKey) {
  return getInitialValueCommon(props, propKey, defaultStateValues)
}
Returns the initial value for a state key in the following order:
1. controlled prop, 2. initial prop, 3. default prop, 4. default
value from Downshift.

@param {Object} props Props passed to the hook.
@param {string} propKey Props key to generate the value for.
@returns {any} The initial value for that prop.