Provide better error messaging on null checks

[TheBlueMatt]

Several times we've had nulls creep in from user code and cause us to do something scary like a 0-ptr deref, which leaves users confused and frustrated, eg https://github.com/orgs/lightningdevkit/discussions/1820. We really should instead null check and call jni->FatalError, even though its a bit more code, because it makes users much happier.