chore: fix nullable "released" field (#30)

Author: official-wizard

src/main/kotlin/org/retroachivements/api/data/pojo/game/GetGame.kt

@@ -48,7 +48,7 @@ class GetGame {
         val genre: String,
 
         @SerializedName("Released")
-        val released: String,
+        val released: String?,
 
         @SerializedName("GameTitle")
         val gameTitle: String,

src/main/kotlin/org/retroachivements/api/data/pojo/game/GetGameExtended.kt

@@ -42,7 +42,7 @@ class GetGameExtended {
         val genre: String,
 
         @SerializedName("Released")
-        val released: String,
+        val released: String?,
 
         @SerializedName("IsFinal")
         val isFinal: Int,

src/main/kotlin/org/retroachivements/api/data/pojo/game/GetGameInfoAndUserProgress.kt

@@ -42,7 +42,7 @@ class GetGameInfoAndUserProgress {
         val genre: String,
 
         @SerializedName("Released")
-        val released: String,
+        val released: String?,
 
         @SerializedName("IsFinal")
         val isFinal: Int,

src/main/kotlin/org/retroachivements/api/data/pojo/user/GetUserSummary.kt

@@ -217,7 +217,7 @@ class GetUserSummary {
             val genre: String,
 
             @SerializedName("Released")
-            val released: String,
+            val released: String?,
 
             @SerializedName("IsFinal")
             val isFinal: Int,