Fix ArrayDecimalField clean method

value could be None when the clean method gets called, if so then we
should return None

Author: manuelnaranjo

rest_framework_filters/fields.py

@@ -2,6 +2,9 @@
 
 class ArrayDecimalField(forms.DecimalField):
     def clean(self, value):
+        if value is None:
+            return None
+
         out = []
         for val in value.split(','):
             out.append(super(ArrayDecimalField, self).clean(val))