Xorg: Handle nil :content value

The `Xorg.find_versions` method was recently updated to replace
`match_data[:content].blank?` with `match_data[:content].empty?` but
this is producing an `undefined method 'empty?' for nil` error, as
`:content` is not present when `PageMatch.find_versions` uses cached
content. This updates `Xorg.find_versions` to handle nil `:content`
values in a way that's similar to other `find_versions` methods.

Library/Homebrew/livecheck/strategy/xorg.rb

@@ -136,9 +136,11 @@ module Homebrew
             options:,
             &block
           )
+          content = match_data[:content]
+          return match_data if content.blank?
 
           # Cache any new page content
-          @page_data[generated_url] = match_data[:content] unless match_data[:content].empty?
+          @page_data[generated_url] = content unless cached_content
 
           match_data
         end