ask-password-api: return if read_credential() failed

ask-password-api: return if read_credential() failed

The current code causes assertion in strv_parse_nulstr() if
read_credential() results in an error different from ENXIO or ENOENT
(strace shows I'm getting EACCES):

 # homectl create waldo --real-name=Waldo --disk-size=200M

Before:

 Assertion 's || l <= 0' failed at src/basic/nulstr-util.c:32, function strv_parse_nulstr_full(). Aborting.

After:

 Failed to acquire password: Permission denied

Follow-up-for: 8806bb4bc7fa15d6ca46e81b8d535730209a3b66