[fzf-mux] Remove unnecessary env var from command

bin/fzf-tmux

@@ -107,7 +107,9 @@ fail() {
   exit 1
 }
 fzf=$(which fzf 2> /dev/null) || fail "fzf executable not found"
-envs="FZF_DEFAULT_OPTS=$(printf %q "$FZF_DEFAULT_OPTS") FZF_DEFAULT_COMMAND=$(printf %q "$FZF_DEFAULT_COMMAND")"
+envs=""
+[ -n "$FZF_DEFAULT_OPTS"    ] && envs="$envs FZF_DEFAULT_OPTS=$(printf %q "$FZF_DEFAULT_OPTS")"
+[ -n "$FZF_DEFAULT_COMMAND" ] && envs="$envs FZF_DEFAULT_COMMAND=$(printf %q "$FZF_DEFAULT_COMMAND")"
 
 mkfifo $fifo2
 mkfifo $fifo3