This from page 8 of the MES 30 Gen 1 owner's manual: "Note: Water bowl must always be in place during smoking process, even if there is no water or other liquid in bowl. Bowl will prevent food from dripping onto wood chips."
So, according to Masterbuilt, the water pan is not essential for heat distribution, not essential for the smoking process, and the smoker won't break if used without the water pan in place. My question to Masterbuilt is this: since the wood chip holder is completely covered, and the wood chip loader extends into this covered area, how exactly will food drip "onto wood chips"? I don't see how it can.
But what this means is that if you're using an AMNS or an AMNPS, it's a non-issue if you've tented the tray with foil. What might be an issue is that the distribution of heat and smoke might change without the water pan in plan and so some minor adjustments in therm probe placements, meat foiling, and overall cooking times might need to be made. I foil over the empty water pan and use it simply as a drip pan which also serves to protect the wood pellets on my AMNPS.
But this should answer the question of if it's safe to use the MES without the water pan. I'd say that it is. Whether you use it or not, fill it with juice or water or leave it empty is entirely your choice.