be honest that non-Module root is possible; return None in such cases

The nodes are often created in an ad-hoc way, and their parent is not
   always set. We can't control for that invariant in the constructor,
   since the parent is usually set outside of the constructor. So,
   let's just be honest that we might not have Module as the root.

astroid/nodes/node_ng.py

@@ -21,7 +21,7 @@
     overload,
 )
 
-from astroid import util
+from astroid import nodes, util
 from astroid.context import InferenceContext
 from astroid.exceptions import (
     AstroidError,
@@ -326,17 +326,18 @@ def scope(self) -> nodes.LocalsDictNodeNG:
             raise ParentMissingError(target=self)
         return self.parent.scope()
 
-    def root(self) -> nodes.Module:
+    def root(self) -> nodes.Module | None:
         """Return the root node of the syntax tree.
 
-        :returns: The root node.
+        :returns: The root node. Might be None, if the node has been
+            created ad-hoc, e.g. with nodes.const_factory
         """
         if not (parent := self.parent):
-            return self  # type: ignore[return-value] # Only 'Module' does not have a parent node.
+            return self if isinstance(self, nodes.Module) else None
 
         while parent.parent:
             parent = parent.parent
-        return parent  # type: ignore[return-value] # Only 'Module' does not have a parent node.
+        return parent if isinstance(parent, nodes.Module) else None
 
     def child_sequence(self, child):
         """Search for the sequence that contains this child.