dev-cmd/bump-formula-pr: fix for undetectable version

Currently, `brew bump-formula-pr` errors out without a proper message
when `--version` isn't specified and the formula's version cannot be parsed
from the URL. We fix this by returning early from `check_new_version` when the
detected version is Version::NULL.

Library/Homebrew/dev-cmd/bump-formula-pr.rb

@@ -436,6 +436,9 @@ module Homebrew
       specs[:tag] = tag if tag.present?
       version = Version.detect(url, **specs)
     end
+
+    return if version.null?
+
     check_throttle(formula, version)
     check_closed_pull_requests(formula, tap_remote_repo, args: args, version: version)
   end