Send zero-fill data when there's no input dato to process

i have a message queue and some blocks that generate waveforms from the
message input. The output of these blocks go to the USRP.
I need a block that when there’s no data to process (0 message in queue)
generates zero-filled samples, to keep USRP busy.
I cannot find a similar block inside gnu radio sources. It’s possible?

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Just put a multiplier in there that under normal circumstances is ‘1’,
and when you want to zero-stuff, set the multiplier to ‘0’.