With commit edab619a (gitaly: Fix access checks with transactions and
quarantine environments, 2021-02-05), we started injecting a flag into
Gitaly requests to force-route to the primary Gitaly node in case a hook
environment is set in order to not break access to quarantined objects.
Turns out that this change breaks read distribution though as now all
requests are force-routed to the primary.

The cause of this is trivial enough: the SafeRequestStore returns an
empty hash if it wasn't set up to contain anything. Given that the
checks whether a HookEnv was set up only checked whether there was
something in the SafeRequestStore, they thus always thought that
requests were running in the context of a HookEnv.

Fix the issue by checking that the returned value is non-empty.