fix: serialize null as kdb+ generic null (::) instead of throwing NPE

[belowzeroff]

Summary

A general list received from kdb+ can contain the generic null ::, which this client deserializes to a Java null (e.g. (1;::;3) → Object[]{1L, null, 3L}). Serializing such a list back — or any Object that is null — threw a bare NullPointerException (from n() → Array.getLength(null)), so kdb+ data containing :: could not be round-tripped.

Reproduction

c c = new c();
// v = deserialized (1;::) => Object[]{1L, null}
Object[] v = { Long.valueOf(1), null };
c.serialize(1, v, false); // -> java.lang.NullPointerException

Change

Serialize a null Object as the kdb+ generic null :: (type 101 followed by a 0 byte) — the exact inverse of how it is deserialized — and account for it as 2 bytes in nx(). A null inside a typed array (e.g. String[]) is unaffected and still serializes as that type's null.

Testing

Added testSerializeDeserializeGenericNull, round-tripping a bare null and a general list containing a null element. Full suite green (85 tests, no regressions).

[sshanks-kx]

method 't' is for the type number of the object, so checking null and returning type number 101 should prob be in there. will also allow the subsequent line to w the type number, rather than having it twice. the the type 101 can be checked as per other types.

[belowzeroff]

Good call, thanks. Moved the null check into t() (returns type number 101 for the generic null ::), so w/nx now handle it through the normal type dispatch and the type byte is written once. The serialized bytes are unchanged. Pushed in 6d81e0b.