ddns-scripts: fixes "sed: no previous regexp"

ddns-scripts: fixes "sed: no previous regexp"

Rewritten Pull for #5885

When ran from the command line, the script prints
error messages like below. They are caused by supplying
empty "$password" and "$URL_PASS" for some log messages
like "130822       : Detect local IP on 'interface'".
The fix is to check if the values are not empty before running
through sed.

/etc/init.d/ddns start
sed: no previous regexp

Reported by Marc Benoit <marcb62185@gmail.com>

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>