xo-alloc2: minor streamline RCollector.std_copy_for

For consistency with equivalent RAllocator version

idl/Collector.json5

@@ -315,9 +315,9 @@
         "void * std_copy_for(const T * src) noexcept {",
         "    void * mem = this->alloc_copy_for(src);",
         "    if (mem) {",
-        "        new (mem) T(std::move(*src));",
+        "        return new (mem) T(std::move(*src));",
         "    }",
-        "    return (T *)mem;",
+        "    return nullptr;",
         "}",
         "",
         "/** forward faceted object pointer in place. Defined in GCObject.hpp to avoid #include cycle **/",