The `filtered_args` method added to the `FormulaInstaller` makes a call
`ARGV.formulae`. Unfortunately, `ARGV.formulae` will throw a
`FormulaUnspecifiedError` instead of returning an empty list. This patch
avoids the issue by checking `ARGV.named.empty?` before calling
`ARGV.formulae`.
FixesHomebrew/homebrew#8576.