helm-marked-files-in-dired fails when helm-ff-default-directory is nil

Attempting to use the `helm-marked-files-in-dired` action fails when called from `helm-source-ls-git`, as the value of `helm-ff-default-directory` is nil. Advising `helm-ls-git-ls` to let-bind the value of `helm-ff-default-directory` to `(helm-ls-git-root-dir)` works as a temporary fix.

To reproduce:
Navigate to a valid project and call helm-ls-git-ls
Navigate to the helm-source-ls-git source named "Git files"
Mark 0 or more files
From select action menu, select "Marked files in dired"