Don't require base_sha in DiffRefsType

app/graphql/types/diff_refs_type.rb

@@ -8,7 +8,7 @@ module Types
 
     field :head_sha, GraphQL::STRING_TYPE, null: false,
           description: 'SHA of the HEAD at the time the comment was made'
-    field :base_sha, GraphQL::STRING_TYPE, null: false,
+    field :base_sha, GraphQL::STRING_TYPE, null: true,
           description: 'Merge base of the branch the comment was made on'
     field :start_sha, GraphQL::STRING_TYPE, null: false,
           description: 'SHA of the branch being compared against'

changelogs/unreleased/security-graphql-diff-refs-empty-base-sha.yml

+---
+title: Don't require base_sha in DiffRefsType
+merge_request:
+author:
+type: security

doc/api/graphql/reference/gitlab_schema.graphql

@@ -1585,7 +1585,7 @@ type DiffRefs {
   """
   Merge base of the branch the comment was made on
   """
-  baseSha: String!
+  baseSha: String
 
   """
   SHA of the HEAD at the time the comment was made

doc/api/graphql/reference/gitlab_schema.json

@@ -8386,13 +8386,9 @@
 
               ],
               "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

@@ -263,7 +263,7 @@ Autogenerated return type of DestroySnippet
 
 | Name  | Type  | Description |
 | ---   |  ---- | ----------  |
-| `baseSha` | String! | Merge base of the branch the comment was made on |
+| `baseSha` | String | Merge base of the branch the comment was made on |
 | `headSha` | String! | SHA of the HEAD at the time the comment was made |
 | `startSha` | String! | SHA of the branch being compared against |
 

spec/graphql/types/diff_refs_type_spec.rb

@@ -5,5 +5,9 @@ require 'spec_helper'
 describe GitlabSchema.types['DiffRefs'] do
   it { expect(described_class.graphql_name).to eq('DiffRefs') }
 
-  it { expect(described_class).to have_graphql_fields(:base_sha, :head_sha, :start_sha) }
+  it { is_expected.to have_graphql_fields(:head_sha, :base_sha, :start_sha).only }
+
+  it { expect(described_class.fields['headSha'].type).to be_non_null }
+  it { expect(described_class.fields['baseSha'].type).not_to be_non_null }
+  it { expect(described_class.fields['startSha'].type).to be_non_null }
 end