NO_COLOR: the empty string should not disable color

[rjbs]

According to no-color.org, which is cited as the authority behind the NO_COLOR environment variable:

Command-line software which adds ANSI color to its output by default should check for a NO_COLOR environment variable that, when present and not an empty string (regardless of its value), prevents the addition of ANSI color.

This branch brings the behavior of Term::ANSIColor in line with that spec.

Thanks very much for Term::ANSIColor, I use it often!