dev-cmd/bump-formula-pr: fix undefined method null error

[milas]

Fix a regression from 8b1fcc7.
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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[nandahkrishna]

Good catch, thanks for your contribution to Homebrew, @milas! 🍻