Make register_instructions optional in RunnerSetup

Updates the GraphQL schema to make register_instructions optional in the
RunnerSetup type.

app/graphql/types/ci/runner_setup_type.rb

@@ -8,7 +8,7 @@ module Types
 
       field :install_instructions, GraphQL::STRING_TYPE, null: false,
         description: 'Instructions for installing the runner on the specified architecture'
-      field :register_instructions, GraphQL::STRING_TYPE, null: false,
+      field :register_instructions, GraphQL::STRING_TYPE, null: true,
         description: 'Instructions for registering the runner'
     end
   end

doc/api/graphql/reference/gitlab_schema.graphql

@@ -18310,7 +18310,7 @@ type RunnerSetup {
   """
   Instructions for registering the runner
   """
-  registerInstructions: String!
+  registerInstructions: String
 }
 
 """

doc/api/graphql/reference/gitlab_schema.json

@@ -52875,13 +52875,9 @@
 
               ],
               "type": {
-                "kind": "NON_NULL",
-                "name": null,
-                "ofType": {
-                  "kind": "SCALAR",
-                  "name": "String",
-                  "ofType": null
-                }
+                "kind": "SCALAR",
+                "name": "String",
+                "ofType": null
               },
               "isDeprecated": false,
               "deprecationReason": null

doc/api/graphql/reference/index.md

@@ -2595,7 +2595,7 @@ Autogenerated return type of RunDASTScan.
 | Field | Type | Description |
 | ----- | ---- | ----------- |
 | `installInstructions` | String! | Instructions for installing the runner on the specified architecture |
-| `registerInstructions` | String! | Instructions for registering the runner |
+| `registerInstructions` | String | Instructions for registering the runner |
 
 ### SastCiConfiguration