Get a single character from the user or input stream as a string. If [expr] is omitted, wait until a character is available. If [expr] is 0 or false, only get a character when one is available. Return an empty string otherwise. If [expr] is 1 or true, only check if a character is available, it is not consumed. Return an empty string if no character is available. Otherwise this works like |getchar()|, except that a number result is converted to a string.