WMS - FIX - if request's bbox doesn't touch layer's bbox, the output must be transparent

[Bang Pham Huu]

For example, a layer has bbox with geo domains:

• X: [30:40]
• Y: [50:60]

then, a WMS GetMap request with bbox: xmin,ymin,xmax,ymax = 30,20,40,30 should return a transparent image with this default rasql query:

SELECT  ENCODE ( EXTEND (<[0:0,0:0] 0c>, [0:255,0:255])  NULL VALUES [0] , "image/png", "{\"nodata\": [0]}")