chore(federation): re-enable nilnil lint (#11253)

First round of patches to re-enable some lints from my side.

This PR also refactors the general key fetching code quite a bit due to the way it currently worked
with relying on some values being nil sometimes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11253
Reviewed-by: elle <0xllx0@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: famfo <famfo@famfo.xyz>
Co-committed-by: famfo <famfo@famfo.xyz>

.golangci.yml

@@ -224,9 +224,6 @@ linters:
       - path: models/dbfs/dbfile.go
         linters:
           - nilnil
-      - path: models/forgefed/federationhost_repository.go
-        linters:
-          - nilnil
       - path: models/forgejo_migrations_legacy/v32.go
         linters:
           - nilnil
@@ -284,9 +281,6 @@ linters:
       - path: models/repo/repo.go
         linters:
           - nilnil
-      - path: models/user/user_repository.go
-        linters:
-          - nilnil
       - path: modules/git/commit.go
         linters:
           - nilnil
@@ -380,9 +374,6 @@ linters:
       - path: routers/api/packages/conan/auth.go
         linters:
           - nilnil
-      - path: services/federation/signature_service.go
-        linters:
-          - nilnil
       - path: services/issue/commit.go
         linters:
           - nilnil