docs: clarify the null residual for map chains with unrelated state_updates

Graph-Agents.md

@@ -1345,10 +1345,13 @@ map's `output_key`, see [`map` nodes](#map)).
 Two consequences of the restore step that bite in parallel super-steps:
 
 - A node with **no `state_updates`** writes nothing at all. A map branch
-  chain that never declares a write to the map's `output_key` therefore
+  chain whose nodes declare no `state_updates`/`output_schema` at all
-  fails the map at run time (the key is removed from each item's fork
+  therefore fails the map at run time (the key is removed from each item's
-  before the chain starts, so nothing is inherited), and the validator
+  fork before the chain starts, so nothing is inherited), and the validator
-  warns about it at load time. Always declare what the branch writes.
+  warns about it at load time. A chain node whose `state_updates` name
+  *other* keys leaves `output: null` instead, so a map with the default
+  `output_key: output` collects `null` — one more reason to always declare
+  a named `output_key` and write it explicitly.
 - When `output` was **absent** from state before the super-step, every
   LLM/agent/rag branch that has `state_updates` leaves `output: null`
   behind (the restore falls back to `null`). Two or more such branches