Sparkle: Account for nil content value

`content` can be `nil` when a request doesn't succeed but
`#versions_from_content` expects a `String` value, so we need to
guard against a `nil` value like we do in other strategies.

Library/Homebrew/livecheck/strategy/sparkle.rb

@@ -190,6 +190,7 @@ module Homebrew
 
           match_data.merge!(Strategy.page_content(url))
           content = match_data.delete(:content)
+          return match_data if content.blank?
 
           versions_from_content(content, regex, &block).each do |version_text|
             match_data[:matches][version_text] = Version.new(version_text)