QIR RT: Consider enabling the null callables (until invocation).

[kuzminrobin]

Related: https://github.com/microsoft/qsharp-runtime/issues/975 https://github.com/microsoft/QuantumLibraries/issues/546

Consider moving this assert to here, here, and here. Such that the null callables are created and exist successfully until the moment of invocation.

[bettinaheim]

I think there is also an issue (not asserts, but also a fail in release builds) for null callables when reference and alias counting for the capture is done.