Misleading error messages for missing keys when creating an upload

[lawrence-forooghian]

Submitting an upload without github_base_ref / github_head_ref in the body gives a validation error e.g. 'github_base_ref must be a string'. These error messages are slightly misleading - github_base_ref and github_head_ref are nullable (i.e. you have to provide the key, but the value is allowed to be null).

Internal discussion: https://ably-real-time.slack.com/archives/C030C5YLY/p1652438829327409?thread_ts=1652428652.185689&cid=C030C5YLY

┆Issue is synchronized with this Jira Uncategorised by Unito