You can safely use the amplifier with no load. But keep in mind that if the amp does not have cabinet simulation then it may cause the output to behave differently.
Cabinet simulation uses internal technology to recreate the electronic behavior and sound associated with plugging the instrument into a speaker. In traditional tube technology the electrical characteristics of an amplifier change drastically when they are plugged into an "reactive load" such as a speaker. Quilter amplifiers recreate this effect when they are plugged into a speaker and so when you choose not to plug in one of our older technology products such as a MicroPro, Aviator Gen 1 or Gold, or 101 series, as well as the Pro Block, Tone Block 200 and 201 and Overdrive 200 their electrical behavior changes significantly when you unplug them. (Please note that on equipped models the direct out uses a padded tap directly from the speaker outputs. 101 Mini Head and Reverb have no direct out.)
What does it mean to the average person? A "snappier" upper end range and sharper mid range.
What can I do if I want cabinet simulation on these models?
There are many companies making cabinet simulation pedals and such that can be put inline between the either the Direct output and/or the headphone jack. These may provide just the right capabilities for your product.
You might also consider using the 202 or InterBlock 45 which both have cabinet simulation built in.