supervisor: fix possible None return for auto_approve_time

This can cause exception later on and prevent updater from execution.
6 jobs for v61.1.5.4 in 3 minutes and 13 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Test
passed cppcheck #460686

00:01:01

passed luacheck #460687

00:01:00

passed system-tests #460684

00:01:38

passed system-valgrind-tests #460685

00:03:13

passed unit-tests #460682

00:01:34

passed unit-valgrind-tests #460683

00:02:56