refactor: remove default value for isManga

lib/models/history.dart

@@ -23,7 +23,7 @@ class History {
 
   History({
     this.id = Isar.autoIncrement,
-    this.isManga = true,
+    this.isManga,
     required this.itemType,
     required this.chapterId,
     required this.mangaId,

lib/models/manga.dart

@@ -65,7 +65,7 @@ class Manga {
       required this.name,
       required this.status,
       required this.description,
-      this.isManga = true,
+      this.isManga,
       this.itemType = ItemType.manga,
       this.dateAdded,
       this.lastUpdate,

lib/models/source.dart

@@ -86,7 +86,7 @@ class Source {
       this.versionLast = "0.0.1",
       this.sourceCode = '',
       this.headers = '',
-      this.isManga = true,
+      this.isManga,
       this.itemType = ItemType.manga,
       this.appMinVerReq = "",
       this.additionalParams = "",