lib/pwauth.c: Simplify empty string

lib/pwauth.c: Simplify empty string

And do not set 'clear' to point to the empty string.  After this commit,
'clear' only stores the result of getpass(3).  This will be useful to
change the code to use agetpass().

$ grep '\<clear\>' lib/pwauth.c;
	char *clear = NULL;
		clear = getpass (prompt);
		input = (clear == NULL) ? "" : clear;
		clear = getpass (prompt);
		input = (clear == NULL) ? "" : clear;
	if (NULL != clear) {
		strzero (clear);

Signed-off-by: Alejandro Colomar <alx@kernel.org>