Add nullable return type when checking if OTP is enabled.

Fixes #17
3 jobs for v0.4.1 in 1 minute and 19 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #6033
test:7.1

00:00:32

passed #5968
test:7.3

00:00:46

failed #5967
test:7.1

01:04:58