Set a null value for topic, break the engine

[moz4r]

<set name="topic"></set> somewhere = no more response anymore

workarround is to add in the set

<topic name="">
<category><pattern>#</pattern>
<template><think><set name="topic">default</set></think><srai><star /></srai></template>
</category>
</topic>

Maybe we can set topic to default if topic is null , automatically ?

MyRobotLab/myrobotlab#307