function getMin( arr ) {
	var tmp;
	var i;
	if ( arr.length === 0 ) {
		return null;
	}
	tmp = new Array( arr.length );
	for ( i = 0; i < arr.length; i++ ) {
		tmp[ i ] = min( arr[ i ] );
	}
	return min( tmp );
}
Computes a minimum value.

@private
@param {Array<NumericArray>} arr - input array
@returns {(number|null)} minimum value or null