decoder: handle empty non-string types

For legacy reasons, non-string types support "empty" values, e.g. you can have
an empty int, distinct from NULL. This case used to trigger deserialization errors,
so now it's simply a fallback to NULL. Another option is to introduce a new "empty"
state that a value can be in, but it's semantically no different from NULL,
even though it's serialized in a different way.

Ref: https://github.com/apache/cassandra/blob/7b58b79fe24c612f8bbf7984c325e88496f482d6/doc/native_protocol_v4.spec#L861-L863

Merge tag '2.16.2'

version 2.16.2

Cluster: do not prolong the lifetime of Cluster for PrepareHostHandler

It was supposed to fix a crash when "host up" event interleaves with
the destruction of Cluster object, but it introduced another bug, which
can be observed e.g. in ControlConnectionTests.Integration_Cassandra_FullOutage
integration test. The bug is that `EventLoop` still has working "timer handle"
while being destroyed and, in turn, segfaults.

Merge pull request #17 from jul-stas/release-scripts

Release as `scylla-cpp-driver`

version 2.15.2

fix CASS_INSTALL_HEADER_IN_SUBDIR usage

version 2.15.0

version 2.14.1

Merge pull request datastax#458 from datastax/2.14.0_prep

2.14.0 release preparation

Merge pull request datastax#450 from datastax/2.13.0_release_prep

OSS v2.13.0 release preparations