fix syntax warning for literal

ldvspec/lofardata/views.py

@@ -175,10 +175,10 @@ def fill_unique_nested_dict(data_product, combined_data_products_on_key):
     for key, value in data_product.items():
         if combined_data_products_on_key.get(key) and \
                 value not in combined_data_products_on_key.get(key) and \
-                value is not None and value is not '':
+                value is not None and value != '':
             combined_data_products_on_key[key].append(value)
         else:
-            combined_data_products_on_key[key] = [value] if value is not None and value is not '' else []
+            combined_data_products_on_key[key] = [value] if value is not None and value != '' else []
     return combined_data_products_on_key