fix(requestlist): use default value of sort direction only if valid (#1174)

The Sort Direction was loaded with values from the localStorage, but `undefined` was assigned if no
previous Sort Direction existed, causing the client to send undefined as a string for the Sort
Direction.

fix #1147

overseerr-api.yml

@@ -5448,6 +5448,7 @@ paths:
           schema:
             type: string
             enum: [asc, desc]
+            nullable: true
             default: desc
         - in: query
           name: requestedBy