Update scan-batch-dir to allow for empty columns at end of the Google Sheet.

[bdgregg]

Currently when reading from a Google Sheet if the sheet has columns that are empty for every row the script does not read it into a Pandas data frame correctly.

E.g.

| col1 | col2 | col3 | col4 |
| A    | B    | C    |      |
| D    | E    | F    |      |

The data frame drops the last column and errors out when updating.

One of the other scripts handles this gracefully. Just need to find the fix and apply it to this script.

Workaround:
Move an existing column with fully populated data to be the last column in the sheet.