Account for spacing in MapFont#getWidth(). Fixes BUKKIT-4089

Prior to this commit MapFont#getWidth() did not account for the 1px
spacing inserted by CraftMapCanvas#drawText().

This commit adds the consideration of the 1px spacing per character
while taking care to not consider the last character as it will not
have a 1px space behind it. This commit also ensures the method will
not check a 0-length String.