api: union undefined

Set the option to generate union undefined types for objects that have
optional items. This will allow us to enable `exactOptionalPropertyTypes`
in our tsconfig [1].

[1] This is a recommended setting, see:
https://www.typescriptlang.org/tsconfig/#exactOptionalPropertyTypes

api/config/contentSources.ts

@@ -7,6 +7,7 @@ const config: ConfigFile = {
   outputFile: '../../src/store/contentSourcesApi.ts',
   exportName: 'contentSourcesApi',
   hooks: true,
+  unionUndefined: true,
   filterEndpoints: [
     'createRepository',
     'listRepositories',