add null category image test mock

Author: etpinard

test/image/baselines/axes_category_null.png

@@ -0,0 +1,44 @@
+{
+    "data": [
+        {
+            "x": [
+                1,
+                2,
+                null,
+                4,
+                5
+            ],
+            "y": [
+                1,
+                2,
+                3,
+                4,
+                5
+            ],
+            "connectgaps": false,
+            "uid": "8ac13a"
+        }
+    ],
+    "layout": {
+        "title": "null categories",
+        "xaxis": {
+            "type": "category",
+            "range": [
+                -0.18336673346693386,
+                3.1833667334669338
+            ],
+            "autorange": true
+        },
+        "yaxis": {
+            "type": "linear",
+            "range": [
+                0.7070063694267517,
+                5.292993630573249
+            ],
+            "autorange": true
+        },
+        "height": 450,
+        "width": 1000,
+        "autosize": true
+    }
+}