Trim empty rows when converting xlsx to tsv

With this change, excel to tabby conversion will preserve empty lines
in the middle of the file (maybe a visual separation of sections in a
many-objects file), but truncate empty lines at the end (maybe excel
artefact). This requires double iteration over the rows (first to find
where data ends, then to export), but it seems inexpensive.

This should help situations when excel (or calc) xlsx file preserves
blank lines.

One test data file (tsv) used to test round-tripping is altered to
remove empty lines at the end. So in the end we no longer guarantee
round-tripping these empty lines, but I feel this was a non-feature.

datalad_tabby/io/xlsx.py

@@ -95,4 +95,13 @@ def _sheet2tsv(ws: Worksheet, dest: Path):
             tsvfile,
             delimiter='\t',
         )
-        writer.writerows(ws.iter_rows(values_only=True))
+
+        # find the last nonempty row
+        max_idx = 1
+        for i, row in enumerate(ws.iter_rows(values_only=True)):
+            if any(v is not None for v in row):
+                max_idx = i + 1  # max row is a 1-based index
+
+        # write tsv, truncating empty rows at the end
+        writer.writerows(ws.iter_rows(values_only=True, max_row=max_idx))
+

datalad_tabby/tests/data/demorecord/tabbydemo_files.tsv

@@ -2,7 +2,3 @@ path[POSIX]	size[bytes]	checksum[md5]	url
 raw/adelie.csv	23755	e7e2be6b203a221949f05e02fcefd853	https://portal.edirepository.org/nis/dataviewer?packageid=knb-lter-pal.219.3&entityid=002f3893385f710df69eeebe893144ff
 raw/gentoo.csv	11263	1549566fb97afa879dc9446edcf2015f	https://portal.edirepository.org/nis/dataviewer?packageid=knb-lter-pal.220.3&entityid=e03b43c924f226486f2f0ab6709d2381
 raw/chinstrap.csv	18872	e4b0710c69297031d63866ce8b888f25	https://portal.edirepository.org/nis/dataviewer?packageid=knb-lter-pal.221.2&entityid=fe853aa8f7a59aa84cdd3197619ef462
-
-
-
-