sh: Fix meaningless unquoted expansion

[[ ${VALUE} ]] means "expand the VALUE variable, splitting on spaces,
and pass those words as arguments to [[…]]". So if VALUE was the string
'-n blah' then that would expand to '[[ -n blah ]]' which is meaningful
but not at all what was intended.

Instead it appears this intented to check for a non-empty string.

tmux-thumbs.sh

@@ -15,7 +15,7 @@ function add-boolean-param {
 function add-option-param {
   VALUE=$(tmux show -vg @thumbs-$1 2> /dev/null)
 
-  if [[ ${VALUE} ]]; then
+  if [ -n "${VALUE}" ]; then
     PARAMS+=("--$1=${VALUE}")
   fi
 }