Low: ocf-shellfuncs: Avoid printing empty INFO messages (bsc#1053621)

When monitoring a pgsql database using ocf:heartbeat:pgsql,
each monitor produces two empty INFO messages in the ha logfile.

This is caused by a broken ocf_run in
/usr/lib/ocf/lib/heartbeat/ocf-shellfuncs which runs:

output=`echo "$output" | tr -s ' \t\r\n' ' '`

This causes output always being ' ', even if the called
command has no output.

The following two tests for ! -z "$output" are broken
and causes these empty messages to appear.

Proposed fix is to only run the tr command if $output
actually has content.