Both are incorrect. There is no such thing as 'more empty'. The meaning of empty means that there is nothing inside a particular area or space. Area or space are things that are uncountable. Its a binary kind of answer where its either empty or its not empty. No such thing as more empty, very empty, not so empty or less empty etc. Do you get it? A simple way to say is " it is empty".

Actually, both are technically invalid, as “empty” is not really a gradeable adjective. In real life, we could use either, although “emptier” is more common. As far as using “more” versus using the suffix “-er”, they are both valid with 2-syllable words ending in “y” (pronounced /i/). It’s probably more typical to use the suffix, for example “easier” is more common than “more easy”. On the other hand, if the adjective has a lot of consonants before the “y”, we may want to avoid using the suffix, for example, if you have trouble pronouncing “more healthy”, it’s OK to say “healthier”. Also, rhotic speakers (those who pronounce the R sound in all positions) may say “more clear”, even though “clearer” is considered correct (since “clear” is a one-syllable word).

pretty close. Normally two or more syllables take "more" (I was taught this as a child). However, I have noticed that the ending "y" has such a small semantic load, and the sound is so unstressed that "more" seems ok. I prefer He is happier than before" rather than "he is more happy than before", I somehow prefer "more empty", but it is quite close. Keep in mind that real live languages are not bound by the rules you have been taught. The "rules" you have been taught try to capture in general terms what the speakers actually do. You don't really think that a native speaker debates these, or "follows known rules", do you? I am sure you don't, when in a bad argument, right? The rules are for outsiders to start getting into the flow of the language. I have found that unless the mistake is really gross, the listener will strive to place some real meaning on a phrase, and once having heard that "turn of phrase" sufficiently, it starts sounding normal. One of the classic examples is the following question, whether it is good English or not: what is this for doing? We normally say, What is this for? and we answer This is for cooking. (what happened to the "dummy do"?) Yet, after I had contemplated the sentence for a few days, it started sounding normal! Also, as a child I never said "He is as smart as me" - always said "as good as I". Now I feel odd when I hear "as good as I", yet "as good as he" still sounds slightly better than "as good as him". And believe me, I am a fully educated native English speaker.

Both are valid, though "emptier" sounds more natural.

Sorry, in writing, the word "emtier" looks weird, but that is because I misspelled it. it is ok as "emptier" -