docs(examples): add bar_label_null example

examples/specs/bar_label_null.vl.json

@@ -0,0 +1,22 @@
+{
+  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
+  "description": "A simple bar chart with embedded data.",
+  "data": {
+    "values": [
+      {"a": "A", "b": 28}, {"a": "B", "b": 55}, {"a": "C", "b": null}
+    ]
+  },
+
+  "encoding": {
+    "x": {"field": "a", "type": "nominal", "axis": {"labelAngle": 0}},
+    "y": {"field": "b", "type": "quantitative"}
+  },
+  "layer": [{
+    "mark": {"type": "bar", "invalid": "show"}
+  }, {
+    "mark": {"type": "text", "invalid": "show", "baseline": "bottom"},
+    "encoding": {
+      "text": {"field": "b", "type": "quantitative"}
+    }
+  }]
+}