AP_NavEKF3: Align CORE determination with other methods

[muramura]

The CORE determination is different! CORE, CORE != NULLPTR.
I think it would be better to align them.

[peterbarker]

Our standard is to compare against nullptr as it adds a very small amount of type-safety.

Note that we won't typically move from !pointer to pointer == nullptr just because, but if you're modifying code in the area we will.

[peterbarker]

core == nullptr is our current standard.

This patch moves us away from that standard.

[peterbarker]

Closing this one as it moves us further from our standard