sync: keep every replayGain element's fields

A second <replayGain> element for the same song cleared whatever the
first one set, because each field was assigned unconditionally and an
absent attribute parses to null. No server is known to split the object
across elements, but the merge costs nothing and the alternative loses
fields silently.

Also covers peaks arriving without gains, which the existing cases did
not.

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicXml.kt

@@ -210,10 +210,10 @@ internal class SongDraft(
     }
 
     fun addReplayGain(attrs: Attributes) {
-        trackGain = attrs.floatAttr("trackGain")
-        albumGain = attrs.floatAttr("albumGain")
-        trackPeak = attrs.floatAttr("trackPeak")
-        albumPeak = attrs.floatAttr("albumPeak")
+        attrs.floatAttr("trackGain")?.let { trackGain = it }
+        attrs.floatAttr("albumGain")?.let { albumGain = it }
+        attrs.floatAttr("trackPeak")?.let { trackPeak = it }
+        attrs.floatAttr("albumPeak")?.let { albumPeak = it }
     }
 
     fun toSong(): Song =