fixes bug in test_manager introduced with mpi args

Author: ecoon

tools/testing/test_manager.py

@@ -278,7 +278,8 @@ def _run_test(self, test_name, dry_run, status, testlog):
             if self._np is not None:
                 if self._mpiexec:
                     command.append(str(self._mpiexec['mpiexec']))
-                    if self._mpiexec['mpiexec_global_args'] is not None:
+                    if self._mpiexec['mpiexec_global_args'] is not None and \
+                       self._mpiexec['mpiexec_global_args'] != '':
                         command.append(str(self._mpiexec['mpiexec_global_args']))
                     command.append(str(self._mpiexec['mpiexec_numprocs_flag']))
                     command.append(str(self._np))