uhd: fixed USRP sink tagged stream gap handling

Fixed a tag gap handling issue caused by an IF statement in tag_work. If length_tag is specified in the usrp_sink constructor but no tag is found on the very first sample, usrp_sink will spin forever printing "tG" and it will never send samples. An example of a situation in which this happens is when a block with history inserts samples before the data stream, such as a filter, so the very first length_tag may not be found until several samples later. Removing this check fixed the issue for me, and Martin said it shouldn't affect his tagged commands logic.