Fail closed on invalid return name declarations

tests/test_generate_popular_node_signatures.py

@@ -743,6 +743,30 @@ NODE_CLASS_MAPPINGS = {
         self.assertEqual({}, result["nodes"])
         self.assertEqual("no_static_nodes", result["pack"]["status"])
 
+    def test_string_return_names_declaration_skips_node(self):
+        source = '''
+class StringReturnNamesNode:
+    RETURN_TYPES = ("IMAGE",)
+    RETURN_NAMES = "image"
+
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "image": ("IMAGE",),
+            },
+        }
+
+
+NODE_CLASS_MAPPINGS = {
+    "StringReturnNamesNode": StringReturnNamesNode,
+}
+'''
+        result = self._extract_source(source, "string-return-names-pack")
+
+        self.assertEqual({}, result["nodes"])
+        self.assertEqual("no_static_nodes", result["pack"]["status"])
+
     def test_missing_return_names_is_allowed(self):
         source = '''
 class MissingReturnNamesNode:

tools/generate_popular_node_signatures.py

@@ -420,7 +420,7 @@ def _class_attr(cls, name, env):
         if name in _bound_names(stmt):
             value = _INVALID
     if value is _MISSING:
-        return None
+        return _MISSING
     if value is _INVALID:
         return _INVALID
     return value
@@ -558,8 +558,12 @@ def _signature_from_class(node_type, cls, display, pack_meta, env):
                 required.append(str(name))
 
     output_names = []
-    if isinstance(return_names, (list, tuple)):
+    if return_names is _MISSING:
+        output_names = []
+    elif isinstance(return_names, (list, tuple)):
         output_names = [str(name) for name in return_names]
+    else:
+        return None
 
     return {
         "type": node_type,