[bindings] Use == null() instead of is_null() to avoid ambiguity

[bindings] Use == null() instead of is_null() to avoid ambiguity

When we have a `Trait` wrapped as an `Option<Trait>`, we called
`<*const Trait>.is_null()` which resulted in rustc trying to take
the most braindead option of dereferencing the whole thing and
hitting a recursive dereference since we
`impl Deref<Target=Trait> for Trait` for all our traits.

Instead, we can be explicit and just compare the pointer directly
with `std::ptr::null()` which avoids this.