Describe the defaults of unconnected node inputs (#234)

Behaviors now says what an unconnected input reads and how the node
shows it.

- A typed-in input without an own value reads false, 0, "" or an
  empty list, the same value the inspector shows.
- The node shows an empty text or list as (empty) and an object or
  struct input without a link as (null).

Behaviors.md

@@ -3,7 +3,11 @@ area (the **Scene** tab shows the editable scene, the **Game** tab the running g
 arrows, control flow) and **data** (coloured circles; `number`, `boolean`, `string`,
 `object`, `list`, `struct`, `any`). Event nodes start a pulse; the interpreter follows exec edges and pulls
 data inputs lazily from connected outputs. Unconnected data inputs use the inline value
-shown on the node and editable in the inspector.
+shown on the node and editable in the inspector. Nothing typed in, an input carries the zero of
+its kind (0, false, empty text, an empty list), and the node shows that value beside the name:
+empty text and an empty list read `(empty)`. An unconnected `object` or `struct` input has
+nothing to type in and carries null, which the node shows as `(null)`. Live output values while
+playing use the same two words.
 
 - **Variables** are the behavior's own property schema (`BehaviorAsset.variables`): they
   appear in the inspector, can be overridden per placed instance, and each running instance