Fixes uninstall if formula is unavailable

[sl4m]

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Encountered an issue where a brewed binary does not have an associated formula.

Error: undefined method `pkgetc' for nil:NilClass
Please report this issue:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:84:in `block (3 levels) in uninstall'
/usr/local/Homebrew/Library/Homebrew/keg.rb:363:in `block in lock'
/usr/local/Homebrew/Library/Homebrew/lock_file.rb:31:in `with_lock'
/usr/local/Homebrew/Library/Homebrew/keg.rb:359:in `lock'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:71:in `block (2 levels) in uninstall'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:60:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:60:in `block in uninstall'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:46:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/uninstall.rb:46:in `uninstall'
/usr/local/Homebrew/Library/Homebrew/brew.rb:110:in `<main>'

#7526 seems to have introduced an issue where uninstall will error if the formula is unavailable (exception raised via FormulaUnavailableError). This fix simply ignores the new code in #7526 unless f is defined.

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @sl4m!