Handle null callable targets in cl::function

[vshalygin]

This issue was identified during a Codex code review.

Constructing cl::function from a null function pointer or null member-function pointer currently creates an apparently non-empty function. Invoking it then results in undefined behavior instead of throwing std::bad_function_call.

Empty std::function targets also remain wrapped as apparently non-empty objects.

Normalize supported null and empty callable targets to the empty cl::function state and add regression tests.