fix(_known_hosts_real): fix "nounset" error with empty known_hosts

When known_hosts files are found but do not contain any valid hosts,
COMPREPLY is not updated in the loop, which causes the "nounset" error
in the test `((${#COMPREPLY[@]}))` after the loop.

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>

Author: akinomyoga

bash_completion

@@ -1974,6 +1974,8 @@ _known_hosts_real()
     # If we have known_hosts files to use
     if ((${#kh[@]} + ${#khd[@]} > 0)); then
         if ((${#kh[@]} > 0)); then
+            COMPREPLY+=() # make sure it exists
+
             # https://man.openbsd.org/sshd.8#SSH_KNOWN_HOSTS_FILE_FORMAT
             for i in "${kh[@]}"; do
                 while read -ra tmpkh; do