HBASE-17357 FIX PerformanceEvaluation parameters parsing triggers NPE.

check command name is not null, if null print usage and exit

Signed-off-by: Michael Stack <stack@apache.org>

hbase-server/src/test/java/org/apache/hadoop/hbase/PerformanceEvaluation.java

@@ -2145,6 +2145,12 @@ public class PerformanceEvaluation extends Configured implements Tool {
         throw new IllegalArgumentException("Number of clients must be > 0");
       }
 
+      // cmdName should not be null, print help and exit
+      if (opts.cmdName == null) {
+        printUsage();
+        return errCode;
+      }
+
       Class<? extends Test> cmdClass = determineCommandClass(opts.cmdName);
       if (cmdClass != null) {
         runTest(cmdClass, opts);