First not empty gets a setting for what counts as empty (#206)

The node treated 0, false and an empty text alike, which is right for a
missing behavior parameter but wrong the moment a real 0 travels through
the graph. A setting now says how much counts as empty, so the same node
works for both cases.

- The new parameter "Empty means" has three steps: "Any empty value"
  (the default, the behaviour so far), "Nothing there (0 and false
  count)" and "Only a missing value".
- The default keeps every existing graph as it is, including the common
  setup where an unset animation parameter arrives as an empty text.
- The rule lives in isEmptyValue(value, mode) in values.ts; the node only
  passes the step through. An unknown setting behaves like the default,
  the same way Compare does.
- The node description is shorter because the old one was cut off in the
  inspector.

Findings:

- In the "Nothing there" step a struct is empty only when its fields are
  empty under the same step, so a position (0, 0) stays a real value.

Tests:

- ./check.sh: 142 files, 2113 tests, exit 0 (before 2104).
- 9 new cases in the node catalog table: the edge cases per step (0,
  false, empty text, empty list, position (0, 0), unconnected a) plus an
  unknown setting.
- Two counterproofs hold: letting "Nothing there" act like "Any" drops
  the 3 cases with 0, false and the position; removing the "Only a
  missing value" step drops the 2 cases with the empty text and list.
- Seen in the browser: the node shows "Empty means: Any empty value" and
  the inspector dropdown lists all three steps.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/values.ts

@@ -92,14 +92,22 @@ export function asString(value: Value | undefined): string {
 }
 
 /**
- * Whether a value is the empty one of its kind: no object, 0, '', false, an empty list, or a struct
- * whose fields are all empty – which is how a position (0, 0) arrives.
+ * How much counts as empty: every empty value of its kind, only the ones that carry nothing (0 and
+ * false are values then), or nothing but a missing one.
  */
-export function isEmptyValue(value: Value | undefined): boolean {
+export type EmptyMode = 'any' | 'nothing' | 'null';
+
+/**
+ * Whether a value is the empty one of its kind: no object, 0, '', false, an empty list, or a struct
+ * whose fields are all empty – which is how a position (0, 0) arrives. `mode` narrows that down.
+ */
+export function isEmptyValue(value: Value | undefined, mode: EmptyMode = 'any'): boolean {
   if (value === null || value === undefined) return true;
+  if (mode === 'null') return false;
   if (isEntity(value)) return false;
   if (isValueList(value)) return value.length === 0;
-  if (isStructValue(value)) return Object.values(value).every(isEmptyValue);
+  if (isStructValue(value)) return Object.values(value).every((field) => isEmptyValue(field, mode));
+  if (mode === 'nothing') return value === '';
   return value === 0 || value === '' || value === false;
 }