Don't serialize bundler.rbi

Also don't create sorbet/gems/ folder if would be empty.

gems/sorbet/lib/gem-generator-tracepoint/tracepoint_serializer.rb

@@ -38,7 +38,9 @@ def initialize(files:, delegate_classes:)
       def serialize(output_dir)
         gem_class_defs = preprocess(@files)
 
-        FileUtils.mkdir_p(output_dir)
+        if !gem_class_defs.empty?
+          FileUtils.mkdir_p(output_dir)
+        end
 
         gem_class_defs.each do |gem, klass_ids|
           File.open("#{File.join(output_dir, gem[:gem])}.rbi", 'w') do |f|
@@ -102,6 +104,10 @@ def files_to_gem_class_defs(files)
             next
           end
           next if gem[:gem] == 'ruby'
+          # We're currently ignoring bundler, because we can't easily pin
+          # everyone to the same version of bundler in tests and in CI.
+          # There is an RBI for bundler in sorbet-typed.
+          next if gem[:gem] == 'bundler'
 
           gem_class_defs[gem] ||= {}
           defined.each do |item|