QmlJs: better indentation for switch common cases

Do not add an empty line between common cases in a `switch` block,
according the best practices for javascript style.

Task-number: QTCREATORBUG-18515
Change-Id: I8b665eabb9b1d5b617d608e430e8b9137dc32032
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/libs/qmljs/qmljsreformatter.cpp

@@ -1102,7 +1102,8 @@ protected:
         out("case ", ast->caseToken);
         accept(ast->expression);
         out(ast->colonToken);
-        lnAcceptIndented(ast->statements);
+        if (ast->statements)
+            lnAcceptIndented(ast->statements);
         return false;
     }