don't call process() if args aren't specified

Example output without args:
```
[-1]R15B01:prime:~/src$ grun
java org.antlr.v4.runtime.misc.TestRig GrammarName startRuleName
  [-tokens] [-tree] [-gui] [-ps file.ps] [-encoding encodingname]
  [-trace] [-diagnostics] [-SLL]
  [input-filename(s)]
Use startRuleName='tokens' if GrammarName is a lexer grammar.
Omitting input-filename makes rig read from stdin.
Exception in thread "main" java.lang.NullPointerException
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
	at org.antlr.v4.runtime.misc.TestRig.process(TestRig.java:157)
	at org.antlr.v4.runtime.misc.TestRig.main(TestRig.java:142)
```