add OpenAPI nullable annotation for Location.region

[kayleesk]

Change description

Fixes #1398 by updating serializers.py per suggestion by @FallenDeity.

AI coding assistance disclosure

Orienting myself in codebase/understanding the suggested change.

Contributor check list

• I have written a description of the contribution and explained its motivation.
• [-] I have written tests for my code changes (if applicable).
• I have read and understood the AI Assisted Contribution guidelines.
• I will own this change in production, and I am prepared to fix any bugs caused by my code change.

[pokeapi-machine-user]

A PokeAPI/api-data refresh has started. In ~45 minutes the staging branch of PokeAPI/api-data will be pushed with the new generated data.

The staging branch will be deployed in our staging environment and the entire API will be ready to review.

A Pull Request (master<-staging) will be also created at PokeAPI/api-data and assigned to the PokeAPI Core team to be reviewed. If approved and merged new data will soon be available worldwide at pokeapi.co.