planner: do not fail if non-existent repository is requested

If package was limited to non-existend repository than this specific
line was failing with nil indexing. This now instead of failure reports
warnings and continues.
10 jobs for fix-242 in 7 minutes and 36 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #254990
cppcheck

00:00:40

passed #254991
luacheck

00:00:37

passed #254988
system-tests

00:01:53

passed #254989
system-valgrind-tests

00:04:12

passed #254986
unit-tests

00:01:05

passed #255031
unit-valgrind-tests

00:03:23

failed #255022
unit-valgrind-tests

00:01:03

failed #255002
unit-valgrind-tests

00:01:03

failed #255000
unit-valgrind-tests

00:01:33

failed #254987
unit-valgrind-tests

00:01:01