allow encrypted fields in custom credentials to be empty

awx/main/tests/unit/models/test_credential.py

@@ -15,3 +15,24 @@ def test_unique_hash_with_unicode():
         credential_type_id=42
     )
     assert cred.unique_hash(display=True) == u'Väult (id=🐉🐉🐉)'
+
+
+def test_custom_cred_with_empty_encrypted_field():
+    ct = CredentialType(
+        name='My Custom Cred',
+        kind='custom',
+        inputs={
+            'fields': [{
+                'id': 'some_field',
+                'label': 'My Field',
+                'secret': True
+            }]
+        }
+    )
+    cred = Credential(
+        id=4,
+        name='Testing 1 2 3',
+        credential_type=ct,
+        inputs={}
+    )
+    assert cred.encrypt_field('some_field', None) is None