Also linked with Bug 8434.

Maybe the following change would do the trick:

diff --git a/C4/Context.pm b/C4/Context.pm
index cc93af4..14ce1c7 100644
--- a/C4/Context.pm
+++ b/C4/Context.pm
@@ -1085,7 +1085,7 @@ sub userenv {
     if (defined $var and defined $context->{"userenv"}->{$var}) {
         return $context->{"userenv"}->{$var};
     } else {
-        return;
+        return {};
     }
 }

Need to be confirmed.

Is this still valid?

(In reply to Katrin Fischer from comment #3)
> Is this still valid?

Yes, it could be a code cleaning.

So far we have to test if userenv is defined before accessing a key, for instance:
  C4/Accounts.pm:    my $branchcode = C4::Context->userenv ? C4::Context->userenv->{'branch'} : undef;

My fix would still be valid, but we still need a test:
  C4/Accounts.pm:    my $branchcode = exist C4::Context->userenv->{branch} ? C4::Context->userenv->{'branch'} : undef;

which actually makes less sense.

A better approach could be to have a ->logged_in_user method, to retrieve a Koha::Patron object, representing the logged in user. But it is a quite huge work, and nobody will sign it off, and rebases will be tricky ;)

Lowering severity for now.