Protect function fill_gaps when given an empty list
Protect function fill_gaps when given an empty list

The function fill_gaps uses the function `max` which
raises an exception when given an empty list. Prevent it.

Return an empty list in that case.

Signed-off-by: Alexandre Lavigne <74b357371b721698434ec0643e6a62bbef389d49@gmail.com>

burnash/gspread