Petascope should initialize array with null value instead of 0

[Dimitar Misev]

When creating a coverage, the array that will hold the data is initialized first like this:

INSERT INTO test_1
VALUES <[0:0,0:0,0:0] {0l,0l,0l,0l,0l,0l,0l,0l,0l}> 
TILING ALIGNED [0:47, 0:19, 0:19] TILE SIZE 19200

The problem is that it is initialized with 0s as values. If the coverage has a null value, petascope should initialize it with the null value.