Prevent run_tests.py from running make if the language has no make targets

tools/run_tests/run_tests.py

@@ -637,6 +637,7 @@ if platform.system() == 'Windows':
       for target in targets]
 else:
   def make_jobspec(cfg, targets, makefile='Makefile'):
+    if targets:
       return [jobset.JobSpec([os.getenv('MAKE', 'make'),
                               '-f', makefile,
                               '-j', '%d' % (multiprocessing.cpu_count() + 1),
@@ -644,6 +645,8 @@ else:
                               args.slowdown,
                               'CONFIG=%s' % cfg] + targets,
                              timeout_seconds=30*60)]
+    else:
+      return []
 make_targets = {}
 for l in languages:
   makefile = l.makefile_name()