When I run a post operation which do a create operation on the server, the latency of the web request is always 0 no matter how big the operation is. 
I traced into the source file, and found in HTTPSamplerBase.java, if no response data, the sampleResult.latencyEnd(); will not be called. 

while ((bytesRead = in.read(readBuffer)) > -1) {
    		if (first) {
    			sampleResult.latencyEnd();
    			first = false;
    		}
    		if (asMD5 && md != null) {
    		    md.update(readBuffer, 0 , bytesRead);
    		    totalBytes += bytesRead;
    		} else {
    		    w.write(readBuffer, 0, bytesRead);
    		}
    	}

Add following code after the while loop will fix this bug.
if (first)
    sampleResult.latencyEnd();