Fix comma when an arrow function with no arguments breaks (#210)

Fixes #118

src/printer.js

@@ -1737,6 +1737,10 @@ function printFunctionParams(path, print, options) {
     printed.push(concat([ "...", path.call(print, "rest") ]));
   }
 
+  if (printed.length === 0) {
+    return "()";
+  }
+
   return concat([
     "(",
     indent(

tests/arrows/__snapshots__/jsfmt.spec.js.snap

@@ -49,3 +49,13 @@ function selectBestEffortImageForWidth(
 }
 "
 `;
+
+exports[`test long-call-no-args.js 1`] = `
+"veryLongCall(VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_LONG_CONSTANT, () => {})
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+veryLongCall(
+  VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_LONG_CONSTANT,
+  () => {}
+);
+"
+`;

tests/arrows/long-call-no-args.js

@@ -0,0 +1 @@
+veryLongCall(VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_VERY_LONG_CONSTANT, () => {})