dev-cmd/audit: Fix "undefined method audit_exceptions"

- When running `brew audit` on formulae that aren't in a Homebrew tap
  location (ie, in a directory outside of
  `/usr/local/Homebrew/Library/Taps`), `brew audit` falls over because
  it can't determine which tap a formula is in.

Library/Homebrew/dev-cmd/audit.rb

@@ -180,7 +180,7 @@ module Homebrew
         except:               args.except,
         spdx_license_data:    spdx_license_data,
         spdx_exception_data:  spdx_exception_data,
-        tap_audit_exceptions: f.tap.audit_exceptions,
+        tap_audit_exceptions: f.tap&.audit_exceptions,
         style_offenses:       style_offenses ? style_offenses.for_path(f.path) : nil,
         display_cop_names:    args.display_cop_names?,
         build_stable:         args.build_stable?,