fix: column misalignment when header is empty

[cgrinds]

Calculate the empty row to use for padding instead of assuming an empty rows is always length=2

[olekukonko]

@cgrinds can you add test in cases where the alignment is center ?

[olekukonko]

we are looking at this already, there is also a pull request I intend to look at next week on this issue namely https://github.com/olekukonko/tablewriter/pull/206