Allow an empty color parameter in \columncolor

[tro100]

How about allowing for an empty color parameter to specify no column color.
This would be very useful for defining column types that may be colored, for instance
\newcolumntype{L}[1]{>{\columncolor{#2}}l}
which then can be used also for columns without coloring.
For the use of \columncolor without optional parameters this could be done by changing the definition of \CT@extract as follows (inserted lines marked as inserted)

\def\CT@extract#1\columncolor#2#3\@nil{%
	\if!\noexpand#2%
		\let\CT@column@color\@empty%
	\else%
		\if[\noexpand#2%
			\CT@extractb{#1}#3\@nil%
		\else%
			\if\relax\detokenize{#2}\relax% inserted
				\let\CT@column@color\@empty% inserted
			\else% inserted
				\def\CT@column@color{%
						\CT@color{#2}}%
			\fi% inserted
			\CT@extractd{#1}#3\@nil%
		\fi%
	\fi%
}%

For the use of \columncolor with optional parameters

\def\CT@extractb#1#2]#3{%
	\if\relax\detokenize{#2}\relax% inserted
		\let\CT@column@color\@empty% inserted
	\else% inserted
		\def\CT@column@color{%
			\CT@color[#2]{#3}}%
	\fi% inserted
	\CT@extractd{#1}%
}%

should do the trick (although I haven't tested this).

Best regards
Rene