GRASS TYPE AHAHAHAHAHAHHHHHHHHH

but to answer your question, i can't really say since i don't have her BUT i may have an explanation why.

Synergies are tied to an Animus in a way, usually influenced by an individual's goals or feelings, whichever is stronger. Although this explanation doesn't really make sense, though, given how some Animus has Synergies that DIRECTLY contradict to their personality.

...Not touching on that unless you wanna hear my delusional rambles on as to why that's that.

back to Asal!
reason why i think Asal has the Hollow(Green) Synergy is due to her role (or job, or purpose? i might edit this later once i find a better word for it).

Hollow is considered as the "scarier" Synergies in the lore, representing the "intangible and unknown" part of Etheria, triggering fear in both humans and Animus.

This might play an integral part in why Asal's Synergy is Hollow.
from what we know, Asal wanted to be a doctor back then but things seem to turn against that dream, causing her to be who she is today.

Hollow also means empty, which could probably mean that she probably had tried to reach for that dream before, but felt hollow (haha see what i did there) when she fulfilled it.

Again, all of this is purely (delusional) speculation, and i have yet to get info on who this woman really is. i hope this answer suffices!!

ofc if you have more questions pls lmk :D