Configure an oauthbearer source without a token_endpoint
parameter would panic due to nil pointer dereference
Example
source=imaps+oauthbearer://frode.aa%40gmail.com@imap.gmail.com:993
source-cred-cmd=pass oatuh2 frode.aa@gmail.com
token_endpoint is not required as it will use the provided
password as access_token when it is not set