The following code returns deterministically the wrong result:
MariaDB return '2', Postgres and MySQL 8.0.16 return '0', which is correct (COUNT should return 0). Therefore, I expect that MariaDB also returns 0.
This bug required DEFAULT 1 and WHERE FALSE. If one removes one of both, the bug disappears.
Additionally, if COUNT is replaced with a constant, all databases return the same result. Therefore, I assume that COUNT forces an evaluation on the empty table, but returns the wrong result (1 instead of 0)
I tested versions 10.4.5 and 10.4.6, both versions are affected. In MySQL 8.0.16, the correct value is returned.