all line attributes should be converted. (#3985)

src/oscar/apps/basket/migrations/0010_convert_to_valid_json.py

 
 def forward(apps, schema_editor):
     LineAttribute = apps.get_model("basket", "LineAttribute")
-    for at in LineAttribute.objects.filter(value__isnull=False):
+    for at in LineAttribute.objects.all():
         try:  # if the value is allready valid json, continue
             json.loads(at.value)
             continue

src/oscar/apps/order/migrations/0012_convert_to_valid_json.py

 
 def forward(apps, schema_editor):
     LineAttribute = apps.get_model("order", "LineAttribute")
-    for at in LineAttribute.objects.filter(value__isnull=False):
+    for at in LineAttribute.objects.all():
         try:  # if the value is allready valid json, continue
             json.loads(at.value)
             continue