TS-4862: Fix metrics clearing.
TS-4862: Fix metrics clearing.

Fix the StatsReset API used by "traffic_ctl metrics clear". If the
name is empty we need to clear all the metrics. The generic management
message marshalling sends NULL as an empty string.

chitianhao/trafficserver,reveller/trafficserver,rahmalik/trafficserver,chitianhao/trafficserver,clearswift/trafficserver,rpufky/trafficserver,chenglongwei/trafficserver,rpufky/trafficserver,persiaAziz/trafficserver,chenglongwei/trafficserver,pbchou/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,rahmalik/trafficserver,taoyunxing/trafficserver,chenglongwei/trafficserver,rpufky/trafficserver,duke8253/trafficserver,clearswift/trafficserver,rahmalik/trafficserver,dyrock/trafficserver,taoyunxing/trafficserver,duke8253/trafficserver,reveller/trafficserver,rahmalik/trafficserver,SolidWallOfCode/trafficserver,davidbz/trafficserver,duke8253/trafficserver,duke8253/trafficserver,PSUdaemon/trafficserver,rpufky/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,taoyunxing/trafficserver,dyrock/trafficserver,rpufky/trafficserver,taoyunxing/trafficserver,vmamidi/trafficserver,chenglongwei/trafficserver,SolidWallOfCode/trafficserver,PSUdaemon/trafficserver,PSUdaemon/trafficserver,reveller/trafficserver,rpufky/trafficserver,reveller/trafficserver,bryancall/trafficserver,davidbz/trafficserver,SolidWallOfCode/trafficserver,taoyunxing/trafficserver,taoyunxing/trafficserver,PSUdaemon/trafficserver,pbchou/trafficserver,persiaAziz/trafficserver,dyrock/trafficserver,chenglongwei/trafficserver,taoyunxing/trafficserver,davidbz/trafficserver,dyrock/trafficserver,PSUdaemon/trafficserver,rahmalik/trafficserver,pbchou/trafficserver,dyrock/trafficserver,pbchou/trafficserver,chenglongwei/trafficserver,duke8253/trafficserver,reveller/trafficserver,clearswift/trafficserver,clearswift/trafficserver,vmamidi/trafficserver,reveller/trafficserver,davidbz/trafficserver,reveller/trafficserver,pbchou/trafficserver,chitianhao/trafficserver,duke8253/trafficserver,SolidWallOfCode/trafficserver,clearswift/trafficserver,taoyunxing/trafficserver,bryancall/trafficserver,PSUdaemon/trafficserver,dyrock/trafficserver,persiaAziz/trafficserver,clearswift/trafficserver,rahmalik/trafficserver,clearswift/trafficserver,bryancall/trafficserver,rpufky/trafficserver,rpufky/trafficserver,vmamidi/trafficserver,persiaAziz/trafficserver,PSUdaemon/trafficserver,PSUdaemon/trafficserver,chitianhao/trafficserver,rahmalik/trafficserver,rahmalik/trafficserver,davidbz/trafficserver,reveller/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,rpufky/trafficserver,chenglongwei/trafficserver,persiaAziz/trafficserver,bryancall/trafficserver,taoyunxing/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,reveller/trafficserver,SolidWallOfCode/trafficserver,persiaAziz/trafficserver,davidbz/trafficserver,chitianhao/trafficserver,rahmalik/trafficserver,persiaAziz/trafficserver,clearswift/trafficserver,PSUdaemon/trafficserver,chenglongwei/trafficserver,chitianhao/trafficserver,chitianhao/trafficserver,clearswift/trafficserver,persiaAziz/trafficserver,dyrock/trafficserver