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>

persistence/persistence_suite_test.go

@@ -12,6 +12,7 @@ import (
 	"github.com/navidrome/navidrome/model"
 	"github.com/navidrome/navidrome/model/request"
 	"github.com/navidrome/navidrome/tests"
+	"github.com/navidrome/navidrome/utils/gg"
 	. "github.com/onsi/ginkgo/v2"
 	. "github.com/onsi/gomega"
 	"github.com/pocketbase/dbx"
@@ -79,7 +80,7 @@ var (
 	songAntenna       = mf(model.MediaFile{ID: "1004", Title: "Antenna", ArtistID: "2", Artist: "Kraftwerk",
 		AlbumID:     "103",
 		Path:        p("/kraft/radio/antenna.mp3"),
-		RGAlbumGain: 1.0, RGAlbumPeak: 2.0, RGTrackGain: 3.0, RGTrackPeak: 4.0,
+		RGAlbumGain: gg.P(1.0), RGAlbumPeak: gg.P(2.0), RGTrackGain: gg.P(3.0), RGTrackPeak: gg.P(4.0),
 	})
 	songAntennaWithLyrics = mf(model.MediaFile{
 		ID:       "1005",