csv_metadata_quality/fix.py: fix logic error again

It seems there was another logic error raised by the test in pytest.
With my real data, it was enough to check if the region column was
None, but with my test I was explicitly setting the region to "" (an
empty string). So to be really sure we should check if the string
is not None *and* if its length is greater than 0.

csv_metadata_quality/fix.py

@@ -361,10 +361,11 @@ def countries_match_regions(row):
                 )
 
         # Add the missing regions back to the row, paying attention to whether
-        # or not the row's regions are blank or not.
-        if row[region_column_name] is not None:
-            row[region_column_name] = row[region_column_name] + "||".join(
-                missing_regions
+        # or not the row's region column is None (aka null) or just an empty
+        # string (length would be 0).
+        if row[region_column_name] is not None and len(row[region_column_name]) > 0:
+            row[region_column_name] = (
+                row[region_column_name] + "||" + "||".join(missing_regions)
             )
         else:
             row[region_column_name] = "||".join(missing_regions)