Treat NO_COLOR="" the same as unset

The docs say

> 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.

but we were not checking for the empty string.

Link: https://no-color.org/
Link: sharkdp/fd#1421

src/parse.c

@@ -3670,7 +3670,8 @@ struct bfs_ctx *bfs_parse_cmdline(int argc, char *argv[]) {
 	}
 
 	enum use_color use_color = COLOR_AUTO;
-	if (getenv("NO_COLOR")) {
+	const char *no_color = getenv("NO_COLOR");
+	if (no_color && *no_color) {
 		// https://no-color.org/
 		use_color = COLOR_NEVER;
 	}

tests/bfs/nocolor_env_empty.out

@@ -0,0 +1,27 @@
+$'rainbow/\e[1m'
+$'rainbow/\e[1m/'$'\e[0m'
+rainbow
+rainbow/exec.sh
+rainbow/socket
+rainbow/broken
+rainbow/chardev_link
+rainbow/link.txt
+rainbow/sticky_ow
+rainbow/sgid
+rainbow/pipe
+rainbow/ow
+rainbow/sugid
+rainbow/suid
+rainbow/sticky
+rainbow/file.dat
+rainbow/file.txt
+rainbow/lower.gz
+rainbow/lower.tar
+rainbow/lower.tar.gz
+rainbow/lu.tar.GZ
+rainbow/mh1
+rainbow/mh2
+rainbow/ul.TAR.gz
+rainbow/upper.GZ
+rainbow/upper.TAR
+rainbow/upper.TAR.GZ

tests/bfs/nocolor_env_empty.sh

@@ -0,0 +1,5 @@
+command -v unbuffer &>/dev/null || skip
+
+NO_COLOR= bfs_pty rainbow >"$OUT"
+sort_output
+diff_output