Fixing image size when returning WMS maps consisting of only empty tiles

In WMS it was treated as an error when only empty tiles were hit, normal error handling ensued. When configured, the errors resulted in empty tiles to be returned to the client. Unfortunately the size of the tiles might not be the requested size, which might cause troubles in some clients.

Also, the behavior to return an error is inconsistent with the behavior in WMTS where empty tiles are returned as empty images.

This Pull Request returns an empty image of the requested size when at least one tile is hit.