strpos behavior change around empty substring in PG12

Greetings hackers,

Before PG12, select strpos('test', '') returns 1 (empty substring found at
first position of the string), whereas starting with PG12 it returns 0
(empty substring not found).

Is this behavior change intentional? If so, it doesn't seem to be
documented in the release notes...

First raised by Austin Drenski in
https://github.com/npgsql/efcore.pg/pull/1068#issuecomment-546795826

Thanks,

Shay