sync_default_gems.rb: do not add extra empty lines [ci skip]

tool/sync_default_gems.rb

@@ -422,7 +422,7 @@ def message_filter(repo, sha)
   log = STDIN.read
   log.delete!("\r")
   url = "https://github.com/#{repo}"
-  subject, log = log.split("\n\n", 2)
+  subject, log = log.split(/\n(?:[\s\t]*(?:\n|\z))/, 2)
   conv = proc do |s|
     mod = true if s.gsub!(/\b(?:(?i:fix(?:e[sd])?) +)\K#(?=\d+\b)|\bGH-#?(?=\d+\b)|\(\K#(?=\d+\))/) {
       "#{url}/pull/"
@@ -440,7 +440,7 @@ def message_filter(repo, sha)
     end
   end
   url = "#{url}/commit/#{sha[0,10]}\n"
-  if log
+  if log and !log.empty?
     conv[log]
     log.sub!(/\s*(?=(?i:\nCo-authored-by:.*)*\Z)/) {
       "\n\n#{url}"