function isValid( value ) {
	return ( isInteger( value ) || isNull( value ) || isUndefined( value ) );
}
Tests whether an input argument is valid.

@private
@param {*} value - value to test
@returns {boolean} boolean indicating whether the argument is valid

@example
var bool = isValid( 3 );
// returns true

bool = isValid( null );
// returns true

bool = isValid( void 0 );
// returns true

bool = isValid( '3' );
// returns false