Make a more logical error when the command/shell module is used with no arguments.

library/command

@@ -35,6 +35,8 @@ def main():
     chdir = module.params['chdir']
     args  = module.params['args']
 
+    if args.strip() == '':
+        module.fail_json(msg="no command given")
 
     if chdir:
         os.chdir(chdir)