Milas Bowman 3364e519b5 dev-cmd/bump-formula-pr: fix undefined method null error
Fix a regression from 8b1fcc7c5c18ec970c30e712ab283d7703694523.
Only if `version.nil?` is `Version.detect` used, so we can
only use `version.null?` after that. (Otherwise, it's a string.)

```
Error: undefined method `null?' for "X.Y.Z":String
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:440:in `check_new_version'
/usr/local/Homebrew/Library/Homebrew/dev-cmd/bump-formula-pr.rb:131:in `bump_formula_pr'
/usr/local/Homebrew/Library/Homebrew/brew.rb:110:in `<main>'
```
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