Import patch to bugfix from Padrino Helpers

Padrino had an issue https://github.com/padrino/padrino-framework/issues/1582
which was solved at 72769fe39a (diff-526024418d0d573e08d46ba3d55c02bcR36)

This commit brings the same fix back into Middleman's method that
overrides Padrino's method.

middleman-core/lib/middleman-more/core_extensions/default_helpers.rb

@@ -12,7 +12,7 @@ class Padrino::Helpers::OutputHelpers::ErbHandler
     raw = block.call(*args)
     captured = template.instance_variable_get(:@_out_buf)
     self.output_buffer = _buf_was
-    engine_matches?(block) ? captured : raw
+    engine_matches?(block) && !captured.empty? ? captured : raw
   end
 end