Only show `kext` caveat on High Sierra or above.

Library/Homebrew/cask/lib/hbc/artifact/abstract_uninstall.rb

@@ -31,7 +31,6 @@ module Hbc
         directives[:signal] = [*directives[:signal]].flatten.each_slice(2).to_a
         @directives = directives
 
-        return if MacOS.version < :high_sierra
         return unless directives.key?(:kext)
 
         cask.caveats do

Library/Homebrew/cask/lib/hbc/dsl/caveats.rb

@@ -17,7 +17,8 @@ module Hbc
       def self.caveat(name, &block)
         define_method(name) do |*args|
           key = [name, *args]
-          @built_in_caveats[key] = instance_exec(*args, &block)
+          text = instance_exec(*args, &block)
+          @built_in_caveats[key] = text if text
           :built_in_caveat
         end
       end
@@ -42,6 +43,7 @@ module Hbc
       end
 
       caveat :kext do
+        next if MacOS.version < :high_sierra
         <<~EOS
           To install and/or use #{@cask} you may need to enable their kernel extension in