Depends on what you define as empty? 0 psi or a low enough pressure to allow water and or moisture can be a problem causing corrosion. Rust corrosion ruins your tank and depletes the oxygen in your tank. Moist enviroment is a incubator for mold mildew and other nasty stuff not good to breathe. If the tank allways has more pressure than the pressure on the outside of the tank I do not see how there would be a problem. But if it is empty or very low you might have to pay for a new viz due to contamination and safety concerns.
Just re read your post and it sounds like you just had your tanks hydoed and or viz with a place that does fire extinguishers. I have done the same thing to save money. The problem is if you do not get a fill right away there is the possibility of moisture that has not been completely removed or moisture and or corosion quickly accuring. Different dive shops may be more cautious then others some do not accept or like the viz from others particular non dive shops.
Good luck hope you have a honest dive shop that treats you fair!