Check array for NULL elements
Char_Type[] = _isnull (a[])
This function may be used to test for the presence of NULL elements
of an array. Specifically, it returns a Char_Type array of
with the same number of elements and dimensionality of the input
array. If an element of the input array is NULL, then the
corresponding element of the output array will be set to 1,
otherwise it will be set to 0.
Set all NULL elements of a string array A to the empty
string "":
A[where(_isnull(A))] = "";
It is important to understand the difference between A==NULL
and _isnull(A). The latter tests all elements of A
against NULL, whereas the former only tests A itself.
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