fix(subsonic): keep the placeholder for a missing username when Gravatar is off

Parsing username before the EnableGravatar short-circuit turned a request with
no username into Subsonic error 10, where it used to return a 200 placeholder
without touching the DB.

server/subsonic/media_retrieval.go

@@ -23,6 +23,11 @@ func (api *Router) GetAvatar(w http.ResponseWriter, r *http.Request) (*responses
 	p := req.Params(r)
 	username, err := p.String("username")
 	if err != nil {
+		// Same reason as the unresolvable-user case below: the old handler short-circuited on
+		// EnableGravatar before it ever looked at the parameter.
+		if !conf.Server.EnableGravatar {
+			return api.getPlaceHolderAvatar(w, r)
+		}
 		return nil, err
 	}
 	ctx := r.Context()

server/subsonic/media_retrieval_test.go

@@ -21,6 +21,7 @@ import (
 	"github.com/navidrome/navidrome/model"
 	"github.com/navidrome/navidrome/model/request"
 	"github.com/navidrome/navidrome/tests"
+	"github.com/navidrome/navidrome/utils/req"
 	. "github.com/onsi/ginkgo/v2"
 	. "github.com/onsi/gomega"
 )
@@ -262,6 +263,23 @@ var _ = Describe("MediaRetrievalController", func() {
 			Expect(err).ToNot(HaveOccurred())
 			Expect(w.Code).To(Equal(http.StatusOK))
 		})
+
+		It("serves the placeholder when the username is missing and Gravatar is off, as before", func() {
+			conf.Server.EnableGravatar = false
+
+			_, err := router.GetAvatar(w, newGetRequest())
+
+			Expect(err).ToNot(HaveOccurred())
+			Expect(w.Code).To(Equal(http.StatusOK))
+		})
+
+		It("returns the same error as before for a missing username when Gravatar is on", func() {
+			conf.Server.EnableGravatar = true
+
+			_, err := router.GetAvatar(w, newGetRequest())
+
+			Expect(err).To(MatchError(req.ErrMissingParam))
+		})
 	})
 
 	Describe("GetLyrics", func() {