[simple-linked-list] Add missing initialValues field

Templating languages that distinguish between undefined and empty list
will crash out on this missing field, or force the template author to
handle the field's absence explicitly.

Author: senekor

exercises/simple-linked-list/canonical-data.json

@@ -185,6 +185,22 @@
           },
           "expected": {}
         },
+        {
+          "uuid": "f3197f0a-1fea-45a5-939f-4a5ea60387ec",
+          "reimplements": "70d747a1-2e84-4ebc-bc3f-dcbee6a05f6b",
+          "description": "Can push to an empty list",
+          "property": "list",
+          "input": {
+            "initialValues": [],
+            "operations": [
+              {
+                "operation": "push",
+                "value": 1
+              }
+            ]
+          },
+          "expected": {}
+        },
         {
           "uuid": "391e332e-1f91-4033-b1e0-0e0c17812fa7",
           "description": "Can push to a non-empty list",