schemagen: keep null elements in arrays with nullable item types

For array types whose items are nullable ({"type": ["T", "null"]}),
the generated builder previously called valueComplete() on cbNull() without
appending anything to the owning vector. Null entries were silently dropped,
so vector indices no longer matched JSON array indices.

Generate isArrayKind() / appendNull() helpers and have cbNull() append a
default-constructed element when the current frame is an array. For
std::optional<T> items this appends an empty optional; for std::unique_ptr<T>
items it appends a null pointer. Add nullable string/integer array fields to
the example schema and test coverage to verify indices are preserved.

Fixes #5

contrib/schemagen/example.schema.json

@@ -71,6 +71,24 @@
     },
     "tree": {
       "$ref": "#/$defs/Node"
+    },
+    "nullable_hobbies": {
+      "type": "array",
+      "items": {
+        "type": [
+          "string",
+          "null"
+        ]
+      }
+    },
+    "nullable_scores": {
+      "type": "array",
+      "items": {
+        "type": [
+          "integer",
+          "null"
+        ]
+      }
     }
   },
   "$defs": {