check: Fix error condition in parsing Flutter version from pubspec

This is intended to check if the pattern failed to match, by
checking whether the array `parsed` has zero elements.  To do this,
it looks at the length of the array... but then it was checking if
that length, as a string, is empty.  Oops.  Instead, check if the
length, as a number, is zero.

Author: gnprice

tools/check

@@ -242,7 +242,7 @@ run_flutter_version() {
              print "$1 $2" if (
                  /^  sdk: .*\n  flutter: '\''>=(\S+)'\''\s*# ([0-9a-f]{40})$/m)'
     ) ) || return
-    if [ -z "${#parsed[@]}" ]; then
+    if (( ! "${#parsed[@]}" )); then
         echo >&2 "error: Flutter version spec not recognized in pubspec.yaml"
         return 1
     fi