Disallowed empty strings for PickledObjectField.

That enforces the usage of `null=True` to store empty values.

README.rst

@@ -168,6 +168,10 @@ Changes in version 2.0.0
 * Silenced ``RemovedInDjango30Warning`` warnings on Django 2.0+ (thanks to
   canarduck).
 * Restructured project directories.
+* Disallowed the usage of empty strings for ``PickledObjectField``. That makes
+  ``.save()``, ``.create()``, etc. raise ``IntegrityError`` if `null` is not
+  ``True`` and no default value was specified like built-in fields do
+  (thanks to Attila-Mihaly Balazs).
 
 Changes in version 1.1.0
 ========================

picklefield/fields.py

@@ -87,6 +87,7 @@ class PickledObjectField(models.Field):
     can still do lookups using None). This way, it is still possible to
     use the ``isnull`` lookup type correctly.
     """
+    empty_strings_allowed = False
 
     def __init__(self, *args, **kwargs):
         self.compress = kwargs.pop('compress', False)

tests/tests.py

@@ -1,6 +1,7 @@
 import json
 from datetime import date
 
+from django.db import IntegrityError
 from django.core import serializers
 from django.test import TestCase
 from picklefield.fields import dbsafe_encode, wrap_conflictual_object
@@ -37,7 +38,7 @@ def test_data_integrity(self):
 
         # Make sure the default value for default_pickled_field gets stored
         # correctly and that it isn't converted to a string.
-        model_test = TestingModel()
+        model_test = TestingModel(pickle_field=1, compressed_pickle_field=1)
         model_test.save()
         model_test = TestingModel.objects.get(id__exact=model_test.id)
         self.assertEqual((D1, S1, T1, L1), model_test.default_pickle_field)
@@ -172,3 +173,7 @@ def test_no_copy(self):
                 compressed_pickle_field='Copy Me',
                 non_copying_field='Dont copy me'
             )
+
+    def test_empty_strings_not_allowed(self):
+        with self.assertRaises(IntegrityError):
+            MinimalTestingModel.objects.create()