refactor(kernel/expr): remove 'null' expression, and operator bool for expression

After this commit, a value of type 'expr' cannot be a reference to nullptr.
This commit also fixes several bugs due to the use of 'null' expressions.

TODO: do the same for kernel objects, sexprs, etc.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/expr): remove 'null' expression, and operator bool for expression