Don't build conf strings if WebJars map is empty

Removing trailing ", " from webjarsVersionsString for an empty webjars map causes StringIndexOutOfBoundsException.

Before:
```
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -2
	at [...]
	at org.webjars.RequireJS.getSetupJavaScript(RequireJS.java:59)
	at [...]
```

After:
```
[main] WARN org.webjars.RequireJS - Can't found any webjar in classpath, RequireJS configuration will be empty.
```