You could use a trigger node to send a blank message after N seconds. Here is an example flow:
[{"id":"6e9b953943096c2f","type":"inject","z":"f4cdc36052e3c33d","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":true,"onceDelay":0.1,"topic":"","payload":"hide me soon","payloadType":"str","x":260,"y":120,"wires":[["3fa949deea8898e1"]]},{"id":"9a4e2f327dfa03b1","type":"debug","z":"f4cdc36052e3c33d","name":"debug 14","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload","targetType":"msg","statusVal":"payload","statusType":"auto","x":680,"y":120,"wires":[]},{"id":"3fa949deea8898e1","type":"trigger","z":"f4cdc36052e3c33d","name":"","op1":"","op2":" ","op1type":"pay","op2type":"str","duration":"2","extend":false,"overrideDelay":false,"units":"s","reset":"","bytopic":"all","topic":"topic","outputs":1,"x":440,"y":120,"wires":[["9a4e2f327dfa03b1"]]}]