Allow null value for mode parameter in getParams function

Explicitly updated the type of the `mode` parameter to `string | null` in the `getParams` function to support cases where `mode` might be `null`. This improves flexibility and handling of optional parameters.

examples/example-umami/lib/get-params.ts

@@ -3,7 +3,7 @@ import { DrupalJsonApiParams } from "drupal-jsonapi-params"
 // A helper function to build params for a resource type.
 export function getParams(
  name: string,
- mode: string = null
+ mode: string | null = null
 ): DrupalJsonApiParams {
  const params = new DrupalJsonApiParams()