Returns an empty UCharString of length length

Property definitions

core $ UCharString :: empty
	# Returns an empty `UCharString` of length `length`
	new empty (length: Int) `{
		UChar * str = (UChar *)malloc(sizeof(UChar) * length);
		u_memset(str, 0, length);
		return str;
	`}
lib/core/text/u16_string.nit:147,2--152,3