fcl-db: bufdataset: when data are written to BLOB field (using TBufBlobStream) then unset field null flag when blob stream is created. When writting finishes (on destroy of blob stream) set null flag if blob is empty. + updated test TestChangeBlob. Bug #15376

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@27089 3ad0048d-3df7-0310-abae-a5850022a9f2