request logger: don't print "empty_string" to logs

The motivation for this patch comes from the request logger logging
"empty_string" for certain annotations. One such annotation is
"darklaunch", which is input as a proplist. An empty proplist is
equivalent to "" in erlang, so the request logger would log
"darklaunch=empty_string" instead of simply ignoring the empty
field.

This patch will make it so that annotations are not logged for
empty strings and lists, matching the behavior for undefined values.