Initializing dynamic arrays to null

The INITIALIZE TO NULL instruction clears the dynamic array.

Starting with version 2.21.00, the INITIALIZE TO NULL instruction clears the dynamic arrays (i.e. array.getLength() returns 0). Before this version, all elements of the dynamic array were kept, and set to null. Since the old behavior was documented, this behavior change required a migration note. The new behavior is expected by most programmers.