Handle the case of **nil in post parameters

Historically, the Sorbet legacy parser has declared bankruptcy in this case
and replaced the entire tree with a `Nil` node. Prism actually handles this
case, which is definitely a better/more complete approach, so I propose
to leave it like this for now and revisit as we're doing more error handling.

parser/prism/Translator.cc

@@ -1033,7 +1033,19 @@ unique_ptr<parser::Node> Translator::translate(pm_node_t *node) {
             if (paramsNode->rest != nullptr)
                 params.emplace_back(translate(paramsNode->rest));
 
-            translateMultiInto(params, posts);
+            // Note: This is actually a divergence from the legacy Sorbet parser behavior, which doesn't handle `**nil`
+            //   as a post argument. In this case, the legacy parser would just replace the entire tree with `Nil`.
+            for (auto &post : posts) {
+                switch (PM_NODE_TYPE(post)) {
+                    case PM_NO_KEYWORDS_PARAMETER_NODE: { // `**nil`
+                        params.emplace_back(translateSimpleKeyword<parser::Kwnilarg>(post));
+                        break;
+                    }
+                    default:
+                        params.emplace_back(translate(post));
+                }
+            }
+
             translateMultiInto(params, keywords);
 
             if (auto prismKwRestNode = paramsNode->keyword_rest; prismKwRestNode != nullptr) {