Make Bing "placeholder" tiles optional

[mike-000]

Is your feature request related to a problem? Please describe.
Currently Aerial and Ordnance Survey styles show placeholder tiles when there is no data because the metadata image url does not include &n=z. The Aerial with Labels image url includes &n=z which produces an empty response instead of a placeholder, and when useInterimTilesOnError and preload layer options are set this causes the nearest available preloaded zoom level to be used instead. If &n=z is removed from the tile requests Aerial with Labels will also display placeholder tiles.

Describe the solution you'd like
This is similar to setting the blankTile=false url option in ESRI XYZ sources to produce an error response instead of a "no information to display" tile. It would be nice if it was as easily configurable for Bing sources (in the same was as &dpi=d1 is added to tile requests for the hidpi option) if a placeholder option was explicitly set true or false (otherwise the metadata defaults would continue to be used).