gh-126105: Fix crash in ast module, when ._fields is deleted (#126115)

Previously, if the `ast.AST._fields` attribute was deleted, attempts to create a new `as`t node would crash due to the assumption that `_fields` always had a non-NULL value. Now it has been fixed by adding an extra check to ensure that `_fields` does not have a NULL value (this can happen when you manually remove `_fields` attribute).

Lib/test/test_ast/test_ast.py

@@ -84,6 +84,23 @@ class AST_Tests(unittest.TestCase):
             # "ast.AST constructor takes 0 positional arguments"
             ast.AST(2)
 
+    def test_AST_fields_NULL_check(self):
+        # See: https://github.com/python/cpython/issues/126105
+        old_value = ast.AST._fields
+
+        def cleanup():
+            ast.AST._fields = old_value
+        self.addCleanup(cleanup)
+
+        del ast.AST._fields
+
+        msg = "type object 'ast.AST' has no attribute '_fields'"
+        # Both examples used to crash:
+        with self.assertRaisesRegex(AttributeError, msg):
+            ast.AST(arg1=123)
+        with self.assertRaisesRegex(AttributeError, msg):
+            ast.AST()
+
     def test_AST_garbage_collection(self):
         class X:
             pass