fix(subsonic): always include mandatory title field in Child responses

Removed `omitempty` from the `Title` struct tag in the `Child` response
type. The Subsonic/OpenSubsonic API spec requires `title` to be a
mandatory field, but songs with empty titles caused the field to be
omitted entirely, crashing clients like Symfonium during sync.

Ref: https://support.symfonium.app/t/app-gets-stuck-on-syncing-large-database/13004/8

server/subsonic/helpers_test.go

@@ -309,6 +309,14 @@ var _ = Describe("helpers", func() {
 				Expect(child.Artist).To(Equal("Test Artist"))
 			})
 		})
+
+		Context("when MediaFile has an empty title", func() {
+			It("still includes the title field in the response", func() {
+				mf.Title = ""
+				child := childFromMediaFile(ctx, mf)
+				Expect(child.Title).To(Equal(""))
+			})
+		})
 	})
 
 	Describe("osChildFromMediaFile", func() {

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

@@ -9,6 +9,7 @@
       {
         "id": "1",
         "isDir": false,
+        "title": "",
         "bpm": 0,
         "comment": "",
         "sortName": "sort name",

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

@@ -1,6 +1,6 @@
 <subsonic-response xmlns="http://subsonic.org/restapi" status="ok" version="1.16.1" type="navidrome" serverVersion="v0.55.0" openSubsonic="true">
   <albumList>
-    <album id="1" isDir="false" sortName="sort name" mediaType="album" musicBrainzId="00000000-0000-0000-0000-000000000000" displayArtist="Display artist" displayAlbumArtist="Display album artist" explicitStatus="explicit">
+    <album id="1" isDir="false" title="" sortName="sort name" mediaType="album" musicBrainzId="00000000-0000-0000-0000-000000000000" displayArtist="Display artist" displayAlbumArtist="Display album artist" explicitStatus="explicit">
       <genres name="Genre 1"></genres>
       <genres name="Genre 2"></genres>
       <moods>mood1</moods>

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

@@ -115,6 +115,7 @@
       {
         "id": "",
         "isDir": false,
+        "title": "",
         "bpm": 0,
         "comment": "",
         "sortName": "",

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

@@ -25,7 +25,7 @@
         <artist id="4" name="composer2"></artist>
       </contributors>
     </child>
-    <child id="" isDir="false">
+    <child id="" isDir="false" title="">
       <replayGain trackGain="0" albumGain="0" trackPeak="0" albumPeak="0" baseGain="0" fallbackGain="0"></replayGain>
     </child>
   </directory>

server/subsonic/responses/.snapshots/Responses Child without data should match .JSON

@@ -8,7 +8,8 @@
     "child": [
       {
         "id": "1",
-        "isDir": false
+        "isDir": false,
+        "title": ""
       }
     ],
     "id": "",

server/subsonic/responses/.snapshots/Responses Child without data should match .XML

@@ -1,5 +1,5 @@
 <subsonic-response xmlns="http://subsonic.org/restapi" status="ok" version="1.16.1" type="navidrome" serverVersion="v0.55.0" openSubsonic="true">
   <directory id="" name="">
-    <child id="1" isDir="false"></child>
+    <child id="1" isDir="false" title=""></child>
   </directory>
 </subsonic-response>

server/subsonic/responses/.snapshots/Responses Child without data should match OpenSubsonic .JSON

@@ -9,6 +9,7 @@
       {
         "id": "1",
         "isDir": false,
+        "title": "",
         "bpm": 0,
         "comment": "",
         "sortName": "",

server/subsonic/responses/.snapshots/Responses Child without data should match OpenSubsonic .XML

@@ -1,5 +1,5 @@
 <subsonic-response xmlns="http://subsonic.org/restapi" status="ok" version="1.16.1" type="navidrome" serverVersion="v0.55.0" openSubsonic="true">
   <directory id="" name="">
-    <child id="1" isDir="false"></child>
+    <child id="1" isDir="false" title=""></child>
   </directory>
 </subsonic-response>

server/subsonic/responses/responses.go

@@ -135,7 +135,7 @@ type Child struct {
 	Id                    string     `xml:"id,attr"                                 json:"id"`
 	Parent                string     `xml:"parent,attr,omitempty"                   json:"parent,omitempty"`
 	IsDir                 bool       `xml:"isDir,attr"                              json:"isDir"`
-	Title                 string     `xml:"title,attr,omitempty"                    json:"title,omitempty"`
+	Title                 string     `xml:"title,attr"                              json:"title"`
 	Name                  string     `xml:"name,attr,omitempty"                     json:"name,omitempty"`
 	Album                 string     `xml:"album,attr,omitempty"                    json:"album,omitempty"`
 	Artist                string     `xml:"artist,attr,omitempty"                   json:"artist,omitempty"`