Fix IndexError with maxheadercolwidths on empty tables

[bysiber]

When maxheadercolwidths is passed and the table has no data rows, tabulate() raises an IndexError because it tries to access list_of_lists[0] to determine the column count on an empty list.

>>> tabulate([], headers=["one", "two", "three"], maxheadercolwidths=5)
IndexError: list index out of range

This fix checks whether list_of_lists is empty and falls back to len(headers) for the column count, which is the same pattern already used in the maxcolwidths handling a few lines above.

>>> tabulate([], headers=["one", "two", "three"], maxheadercolwidths=5)
'one    two    three\n-----  -----  -------'

Fixes #365

[astanin]

Thank you for pointing this out. I did a slightly different fix.