Describe the bug
In taskwarrior configuration file, boolean truthness values can be represented by the following values, case insensitive (from taskrc manpage):

Note that Taskwarrior is flexible about the values used to represent Boolean items. You can use "on", "yes",
"y", "1" and "true". Anything else means "off".

vit only accepts yes (lowercase) to represent truth in print.empty.columns config variable.

To Reproduce

1. Configure print.empty.columns with any of the above mentionned truthness values, case insensitive, except yes (attached test case uses True)
2. Display next report
3. Compare with task next output

=> Columns with no data are not displayed

Expected behavior
Columns with no data are displayed.

Affected code

Something like

self.print_empty_columns = self.subtree('print.empty.columns').lower() in ['y', 'yes', 'on', 'true', '1']

would fix this specific issue.

Test case
print-empty-columns.sh.txt