Optimize (not) printing empty string from formatter

Sometimes it is handy to implement some additional filtering in a
custom formatter function. In this case the formatter can return the
empty string if it wants to ignore a trace message. Optimize the
formatting so in the empty string case it does not send an io request
to the IO server. This avoids a "flickering prompt" effect (when the
empty string is printed) in case of 1000s of ignored trace messages
per second.

src/recon_trace.erl

@@ -393,7 +393,10 @@ formatter(Tracer, IOServer, FormatterFun) ->
         {'EXIT', Tracer, Reason} ->
             exit(Reason);
         TraceMsg ->
-            io:format(IOServer, FormatterFun(TraceMsg), []),
+            case FormatterFun(TraceMsg) of
+                "" -> ok;
+                Formatted -> io:format(IOServer, Formatted, [])
+            end,
             formatter(Tracer, IOServer, FormatterFun)
     end.