FIX - WMS should filter null values if they don't belong to range values: [0-225]

[Bang Pham Huu]

for example, if a collection has null value = -9999 or 9999 which is out of range for encoding as PNG (valid values [0-255]) in WMS GetMap result, then the value should be filtered.

If a collection has 3 bands, and each ban has a different list of null values like below:

{[3], [NaN], [-25.5]}

then in the rasql query for a WMS GetMap request, null values in the encode() parameter is changed to

{[3],[],[]}