hydra-server: findLog: fix issue with ca-derivations enabled

When content addressed derivations are built on the hydra server,
one may run into an issue where some builds suddenly don't load anymore.

This seems to be caused by outPaths that are NULL (which is
allowed for ca-derivations). Filter them out to prevent querying the
database for them, which is not supported by the database abstraction
layer that's currently in use.

On my instance this appears to resolve the issue.
I feel like I might be doing this at the wrong abstraction layer, but on
the other hand -- it seems to resolve it and it also doesn't really look
like it will hurt anything.

The test added in a previous commit uncovers this issue, and this commit
resolves it. So I'm happy with this patch for now.

The issue I was seeing on my server:

hydra-server[2549]: [error] Couldn't render template "undef error - DBIx::Class::SQLMaker::ClassicExtensions::puke(): Fatal: NULL-within-IN not implemented: The upcoming SQL::Abstract::Classic 2.0 will emit the logically correct SQL instead of raising this exception. at /nix/store/<hash>-hydra-unstable-2024-03-08_nix_2_20/libexec/hydra/lib/Hydra/Helper/Nix.pm line 190

See also short discussion here: https://github.com/NixOS/nixpkgs/pull/297392#issuecomment-2035366263

src/lib/Hydra/Helper/Nix.pm

@@ -187,6 +187,10 @@ sub findLog {
 
     return undef if scalar @outPaths == 0;
 
+    # Filter out any NULLs. Content-addressed derivations
+    # that haven't built yet or failed to build may have a NULL outPath.
+    @outPaths = grep {defined} @outPaths;
+
     my @steps = $c->model('DB::BuildSteps')->search(
         { path => { -in => [@outPaths] } },
         { select => ["drvpath"]