Do NOT emit empty log-messages

[c5253458]

Use Case:
export logger_info logger_export; scp -p file host:/tmp/file 1> >(logger_info) 2> >(logger_error)

Situation now: even on success of scp , when nothing is written to logger_error, an error-log entry is written.

Desired: on success of scp , when nothing is written to logger_error, write no error-log entry.

Code-Change:

log() {
...
# if no message was passed, read it from STDIN
[ $# -ne 0 ] && l_msg="$@" || l_msg=cat
if [ "" = "${l_msg}" ]; then return; fi