JavaScript: handle lack of modifiers on extracted method

The emitter expects undefined, rather than empty.  This only affects JS,
because TS applies `private` to all extracted methods.

(cherry picked from commit 9630c46ea7)

src/services/refactors/extractMethod.ts

@@ -664,7 +664,7 @@ namespace ts.refactor.extractMethod {
             }
             newFunction = createMethod(
                 /*decorators*/ undefined,
-                modifiers,
+                modifiers.length ? modifiers : undefined,
                 range.facts & RangeFacts.IsGenerator ? createToken(SyntaxKind.AsteriskToken) : undefined,
                 functionName,
                 /*questionToken*/ undefined,