Add failing test to recreate EVL upload error

If a cell is an empty string, it is correctly removed and there's no
attempt to convert it to an integer. Recently we've seen a sneaky
whitespace which isn't treated the same. This causes an upload failure.
This test modification reproduces the problem.

tests/contrib/test_evl_upload_filters.py

@@ -60,6 +60,7 @@ def test_converts_missing_failure_as_zero(self):
             self.ignore_rows(4) +
             [["No tax-disc failure", 0, '', 0, 2]] +
             [["No sorn failure", 1, 20, 0, '']] +
+            [["Failure with whitespace in value", 1, 20, 0, ' ']] +
             [[None, "None in first column means end of failures list"]]
         ]