Do not fail when asset was already compiled

In the case where a deploy was happening after all the assets were already generated, a future to write a file to disk is not created, (i.e. in the "skipped writing" file code path). We now check for the presence of this future before trying to `wait!` on it.

lib/sprockets/manifest.rb

@@ -203,7 +203,7 @@ def compile(*args)
         else
           logger.info "Writing #{target}.gz"
           concurrent_compressors << Concurrent::Future.execute do
-            write_file.wait!
+            write_file.wait! if write_file
             gzip.compress(target)
           end
         end

test/test_manifest.rb

@@ -640,6 +640,26 @@ def teardown
     end
   end
 
+
+  test "writes gzip files even if files were already on disk" do
+    @env.gzip = false
+    manifest = Sprockets::Manifest.new(@env, @dir)
+    files = %W{ gallery.css application.js logo.svg }
+    files.each do |file_name|
+      original_path = @env[file_name].digest_path
+      manifest.compile(file_name)
+      assert File.exist?("#{@dir}/#{original_path}"), "Expecting \"#{@dir}/#{original_path}\" to exist but did not"
+    end
+
+    @env.gzip = true
+    files.each do |file_name|
+      original_path = @env[file_name].digest_path
+      manifest.compile(file_name)
+      assert File.exist?("#{@dir}/#{original_path}.gz"), "Expecting '#{original_path}' to generate gzipped file: '#{original_path}.gz' but it did not"
+      assert_equal File.stat("#{@dir}/#{original_path}").mtime, Zlib::GzipReader.open("#{@dir}/#{original_path}.gz") {|gz| gz.mtime }
+    end
+  end
+
   test "disable file gzip" do
     @env.gzip = false
     manifest = Sprockets::Manifest.new(@env, @dir)