loader: check for null pointer in getArray() and getString()

[jedisct1]

When NULL pointers are returned, the loader's module getArray() and getString() return arbitrary data sitting at offset 0.

Even though null shouldn't be returned for error handling, this can happen in real code.

Throwing an exception instead prevents undefined behaviors.

[dcodeIO]

I think that instead of throwing, these functions should just return null because the underlying string or array reference type might be nullable. Wdyt?

[jedisct1]

Calling getString() on a null pointer, no matter what the underlying type is, should throw an exception IMHO. Can't think of use case where that wouldn't be due to a bug.

[dcodeIO]

This has been somewhat implemented by means of an id check meanwhile. In general I think that we should strive to avoid potentially redundant checks like the one proposed here so we don't find us in a situation where these add up, and instead document the behavior more thoroughly, so users are aware that if they need to check, they must check.

Closing this PR as part of 2020 vacuum because the loader has changed substantially meanwhile, and creating a new PR documenting the behavior.