Revert "doc/conventions.txt: Switch get_x behavior to be like check_x; TODO: Implement this behavior"

This reverts commit 702528b542a18d9e916ceab89e223190e2814c2b.

Conflicts:
	doc/todo.txt
builtin: Switch to correct get_x() nullptr convention
core/types.h: T* check(T *v)
server/state: Fix file watching and module reload