[install] Fix empty-shell detection in install script

${#shells} is the string length, not the shell count.

Thanks to @matheus-pacifico for the report.

Close #4813

install

@@ -233,7 +233,7 @@ for s in $shells; do
   fi
 done
 
-if [[ ${#shells} -lt 3 ]]; then
+if [[ -z "${shells// /}" ]]; then
   echo "No shell configuration to be updated."
   exit 0
 fi