Add test coverage: don't make id: ID non-nullable on non-indexed types. (#1122)

After merging #1118, I realized that there's an alternate implementation
that would pass the test added in that PR but would be wrong: we could
always set the `id` field of any object type to be non-nullable. We only
want to do so on indexed types.

This adds a test confirming that `id: ID` is left as nullable in on a
non-indexed type.

Author: myronmarston

elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/json_schema_spec.rb

@@ -1140,7 +1140,7 @@ def have_json_schema_like(type_name, *args, **kwargs)
         })
       end
 
-      it "makes the `id` field non-nullable in JSON schema even when declared as a nullable `ID` in the GraphQL schema" do
+      it "makes an `id: ID` field of an indexed type non-nullable in JSON schema" do
         json_schema = dump_schema do |s|
           s.object_type "Widget" do |t|
             t.field "id", "ID"
@@ -1163,6 +1163,27 @@ def have_json_schema_like(type_name, *args, **kwargs)
         )
       end
 
+      it "leaves an `id: ID` field on a non-indexed type as nullable in the JSON schema" do
+        json_schema = dump_schema do |s|
+          s.object_type "Widget" do |t|
+            t.field "id", "ID"
+            t.field "name", "String"
+          end
+        end
+
+        expect(json_schema).to have_json_schema_like("Widget", {
+          "type" => "object",
+          "properties" => {
+            "id" => json_schema_ref("ID"),
+            "name" => json_schema_ref("String")
+          },
+          "required" => %w[id name]
+        }).which_matches(
+          {"id" => "abc", "name" => "thing"},
+          {"id" => nil, "name" => "thing"}
+        )
+      end
+
       context "on an indexed type with a rollover index" do
         it "makes the JSON schema for a rollover index timestamp field on the indexed type non-nullable since a target index cannot be chosen without it" do
           json_schema = dump_schema do |s|