function isEmptyString( prop ) {
	return ( prop === '' );
}
Tests if an indexing expression is an empty string.

@private
@param {string} prop - property name
@returns {boolean} result

@example
var out = isEmptyString( '' );
// returns true

@example
var out = isEmptyString( '...' );
// returns false

@example
var out = isEmptyString( '-2' );
// returns false