fix: Allow nullable ReplayGain and support 0.0 (#4239)

* fix(ui,scanner,subsonic): Allow nullable replaygain and support 0.0

Resolves #4236.

Makes the replaygain columns (track/album gain/peak) nullable.
Converts the type to a pointer, allowing for 0.0 (a valid value) to be returned from Subsonic.
Updates tests for this behavior.

* small refactor

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan <deluan@navidrome.org>

server/subsonic/api_test.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"github.com/navidrome/navidrome/server/subsonic/responses"
+	"github.com/navidrome/navidrome/utils/gg"
 	. "github.com/onsi/ginkgo/v2"
 	. "github.com/onsi/gomega"
 )
@@ -91,7 +92,7 @@ var _ = Describe("sendResponse", func() {
 		It("should return a fail response", func() {
 			payload.Song = &responses.Child{OpenSubsonicChild: &responses.OpenSubsonicChild{}}
 			// An +Inf value will cause an error when marshalling to JSON
-			payload.Song.ReplayGain = responses.ReplayGain{TrackGain: math.Inf(1)}
+			payload.Song.ReplayGain = responses.ReplayGain{TrackGain: gg.P(math.Inf(1))}
 			q := r.URL.Query()
 			q.Add("f", "json")
 			r.URL.RawQuery = q.Encode()