Clean up OpErase implementation

[object-Object]

This feels like a really scuffed way of checking if you can erase something, especially since "contains NULL" and "empty" are distinct states, so "can write NULL" doesn't necessarily imply "can make empty".

https://github.com/FallingColors/HexMod/blob/74bafe20156d68628e21175289f68aaf05707ee3/Common/src/main/java/at/petrak/hexcasting/common/casting/actions/spells/OpErase.kt#L23-L25

[Talia-12]

hmm, I think "write null" (as in java null, not a NullIota()) is how the API currently implements erasing, so it makes sense that checking whether that's possible is how that currently works? but it would be very reasonable to consider "write null to erase" as not great practice