HPCC-9182 Prevent adding empty FlushingStringBuffer block to queue

An empty block in the FlushingStringBuffer queue will be treated
as end of content.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>
HPCC-9182 Make common routine for adding to FlushingStringBuffer queue

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>
HPCC-9182 CSafeSocket write should handle buffer size of 0

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>
Merge pull request #4286 from afishbeck/incompletRoxieXMLBuffers9182

HPCC-9182 Prevent adding empty block to FlushingStringBuffer queue

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>