Dynamic assets

Currently AssetPack does not support dynamic assets, i.e. assets generated by the Mojolicious app itself. The attached patch makes this possible - however it randomly spits out an error message "Use of uninitialized value in hash element at lib/Mojolicious/Plugin/AssetPack.pm line 362.", which I am not able to track down.
Why have you changed this? When do you want to have an error mesage that is 0 or empty string?