fix: Add empty constructor

Need this for REST requests

gemeinsamforschen/src/main/java/unipotsdam/gf/modules/annotation/model/AnnotationBody.java

@@ -12,7 +12,7 @@ public class AnnotationBody {
     private int startCharacter;
     private int endCharacter;
 
-    // constructor
+    // constructors
     public AnnotationBody(String title, String comment, int startCharacter, int endCharacter) {
         this.title = title;
         this.comment = comment;
@@ -20,6 +20,9 @@ public class AnnotationBody {
         this.endCharacter = endCharacter;
     }
 
+    public AnnotationBody() {
+    }
+
     // methods
     public String getTitle() {
         return title;