1. Null filter doesn't ever goes inactive

This was seen using the C throttle test program for lcls on linux. Null filter on text field does filter when it should. But, the filter never resets and lets messages flow again.

  cmlog_slac_set_throttle_string ("text",       /* in field "text"  */
                                  2,            /* allow 2 messages */
                                  10.0,         /* every 10 seconds */
                                  "");          /* matching all defined values for "text" tag */