Remove usage of `to_list_type` in GQL types

- to_list_type is part of the gem's private API and should not be used

app/graphql/resolvers/concerns/issue_resolver_arguments.rb

@@ -12,10 +12,10 @@ module IssueResolverArguments
     argument :iids, [GraphQL::STRING_TYPE],
               required: false,
               description: 'List of IIDs of issues. For example, [1, 2].'
-    argument :label_name, GraphQL::STRING_TYPE.to_list_type,
+    argument :label_name, [GraphQL::STRING_TYPE, null: true],
               required: false,
               description: 'Labels applied to this issue.'
-    argument :milestone_title, GraphQL::STRING_TYPE.to_list_type,
+    argument :milestone_title, [GraphQL::STRING_TYPE, null: true],
               required: false,
               description: 'Milestone applied to this issue.'
     argument :author_username, GraphQL::STRING_TYPE,

app/graphql/types/boards/board_issuable_input_base_type.rb

@@ -4,7 +4,7 @@ module Types
   module Boards
     # Common arguments that we can be used to filter boards epics and issues
     class BoardIssuableInputBaseType < BaseInputObject
-      argument :label_name, GraphQL::STRING_TYPE.to_list_type,
+      argument :label_name, [GraphQL::STRING_TYPE, null: true],
                required: false,
                description: 'Filter by label name.'
 

app/graphql/types/boards/board_issue_input_base_type.rb

@@ -8,7 +8,7 @@ module Types
                required: false,
                description: 'Filter by milestone title.'
 
-      argument :assignee_username, GraphQL::STRING_TYPE.to_list_type,
+      argument :assignee_username, [GraphQL::STRING_TYPE, null: true],
                required: false,
                description: 'Filter by assignee username.'
 

app/graphql/types/issues/negated_issue_filter_input_type.rb

@@ -8,10 +8,10 @@ module Types
       argument :iids, [GraphQL::STRING_TYPE],
                 required: false,
                 description: 'List of IIDs of issues to exclude. For example, [1, 2].'
-      argument :label_name, GraphQL::STRING_TYPE.to_list_type,
+      argument :label_name, [GraphQL::STRING_TYPE],
                 required: false,
                 description: 'Labels not applied to this issue.'
-      argument :milestone_title, GraphQL::STRING_TYPE.to_list_type,
+      argument :milestone_title, [GraphQL::STRING_TYPE],
                 required: false,
                 description: 'Milestone not applied to this issue.'
       argument :assignee_username, GraphQL::STRING_TYPE,

ee/app/graphql/ee/resolvers/issues_resolver.rb

@@ -7,7 +7,7 @@ module EE
       extend ::Gitlab::Utils::Override
 
       prepended do
-        argument :iteration_id, ::GraphQL::ID_TYPE.to_list_type,
+        argument :iteration_id, [::GraphQL::ID_TYPE, null: true],
                  required: false,
                  description: 'Iterations applied to the issue.'
         argument :epic_id, GraphQL::STRING_TYPE,