[th/clicolor-force] support CLICOLOR_FORCE variable and fix handling of empty NO_COLOR

See also:

• https://bixense.com/clicolors/

• https://no-color.org/

• https://github.com/jcs/no_color/commit/99f90e27d0fac6a34d07998f8f577e1f2f620c61

• https://github.com/jhasse/clicolors/issues/13