Fix DiscTitle OpenSubsonic compatibility. Closes #2929

server/subsonic/responses/.snapshots/Responses AlbumWithSongsID3 with data should match .JSON

@@ -29,6 +29,10 @@
       {
         "disc": 2,
         "title": "disc 2"
+      },
+      {
+        "disc": 3,
+        "title": ""
       }
     ],
     "originalReleaseDate": {

server/subsonic/responses/.snapshots/Responses AlbumWithSongsID3 with data should match .XML

@@ -4,6 +4,7 @@
     <genres name="progressive"></genres>
     <discTitles disc="1" title="disc 1"></discTitles>
     <discTitles disc="2" title="disc 2"></discTitles>
+    <discTitles disc="3" title=""></discTitles>
     <originalReleaseDate year="1994" month="2" day="4"></originalReleaseDate>
     <releaseDate year="2000" month="5" day="10"></releaseDate>
     <song id="1" isDir="true" title="title" album="album" artist="artist" track="1" year="1985" genre="Rock" coverArt="1" size="8421341" contentType="audio/flac" suffix="flac" starred="2016-03-02T20:30:00Z" transcodedContentType="audio/mpeg" transcodedSuffix="mp3" duration="146" bitRate="320" isVideo="false" bpm="127" comment="a comment" sortName="sorted song" mediaType="song" musicBrainzId="4321">