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 Job ID Name Coverage
  Test
passed #460686
cppcheck

00:01:01

passed #460687
luacheck

00:01:00

passed #460684
system-tests

00:01:38

passed #460685
system-valgrind-tests

00:03:13

passed #460682
unit-tests

00:01:34

passed #460683
unit-valgrind-tests

00:02:56