Added default constructors to database models

Empty constructors are needed as we apply the single parameters
using setters. Also it seems to be a sqlite standard.

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Accessory.java

@@ -36,6 +36,11 @@ public class Accessory {
      */
     private int mAvailable;
 
+    /**
+     * Empty default constructor.
+     */
+    public Accessory() {}
+
     /**
      * The constructor of this accessory object.
      *

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Character.java

@@ -41,6 +41,11 @@ public class Character {
      */
     private int mAvailable;
 
+    /**
+     * Empty default constructor.
+     */
+    public Character() {}
+
     /**
      * The constructor of this character object.
      *

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Game.java

@@ -31,6 +31,11 @@ public class Game {
      */
     private String mCoverUri;
 
+    /**
+     * Empty default constructor.
+     */
+    public Game() {}
+
     /**
      * The constructor of this game object.
      *

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Kart.java

@@ -62,6 +62,11 @@ public class Kart {
      */
     private int mAvailable;
 
+    /**
+     * Empty default constructor.
+     */
+    public Kart() {}
+
     /**
      * The constructor of this kart object.
      *

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Ruleset.java

@@ -61,6 +61,11 @@ public class Ruleset {
      */
     private int mBikesAvailable;
 
+    /**
+     * Empty default constructor.
+     */
+    public Ruleset() {}
+
     /**
      * The constructor of this ruleset object.
      *

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Track.java

@@ -46,6 +46,11 @@ public class Track {
      */
     private int mAvailable;
 
+    /**
+     * Empty default constructor.
+     */
+    public Track() {}
+
     /**
      * The constructor of this track object.
      *

app/src/main/java/com/de/aldo_apps/aldo/mariokartcircuitselector/database_models/Wheels.java

@@ -36,6 +36,11 @@ public class Wheels {
      */
     private int mAvailable;
 
+    /**
+     * Empty default constructor.
+     */
+    public Wheels() {}
+
     /**
      * The constructor of this wheels object.
      *