compose: handle Modular messages with no compose_id correctly
compose: handle Modular messages with no compose_id correctly

The code for generating links for compose fedmsgs assumed that
messages for Modular composes would always have a `compose_id`,
but in fact they do not: messages sent before pungi-koji runs,
e.g. the `compose.start` message, don't have a `compose_id`
because it is not known at that time. This caused datagrepper
to fail whenever asked to display any such messages.

It's actually a bit of an interesting question what the 'best'
URL to give for such fedmsgs is, but I'm getting a headache
from thinking about it too hard, so for now let's just do this,
we'll return https://kojipkgs.fedoraproject.org/compose/ as
the 'link' for modular compose fedmsg's with no compose_id.

Signed-off-by: Adam Williamson <9343557fe728326ecca60ea600eccabb6754179b@redhat.com>

fedora-infra/fedmsg_meta_fedora_infrastructure