Empty stream should result in empty DB value, not a NULL

Currently there is a testcase ensuring it is NULL. I guess the idea was to make possible to insert NULL value with a stream. But at the same time that makes impossible to insert empty value with a stream. While NULL value can be set with either setNull or nullptr passed to setBlob.
Perhaps the stream also should yield in NULL DB value if it's failing(i.e. istream::fail returns true)