Allow partial-blocks to be executed without "options"

Closes #1341

If the @partial-block is called as parameter of a helper (like in
{{#if @partial-block}}...{{/if}}, the partialBlockWrapper is executed
without "options"-parameter. It should still work in without an error
in such a case.

lib/handlebars/runtime.js

@@ -237,7 +237,8 @@ export function invokePartial(partial, context, options) {
     options.data = createFrame(options.data);
     // Wrapper function to get access to currentPartialBlock from the closure
     let fn = options.fn;
-    partialBlock = options.data['partial-block'] = function partialBlockWrapper(context, options) {
+    partialBlock = options.data['partial-block'] = function partialBlockWrapper(context, options = {}) {
+
       // Restore the partial-block from the closure for the execution of the block
       // i.e. the part inside the block of the partial call.
       options.data = createFrame(options.data);