Issue with `CircuitTracer`

[renatomello]

The interfaces.circuit_tracer.CircuitTracer class has a method called build_circuit. This method has an x argument that defaults to None. x is then passed to another method of CircuitTracer called _build_from_encoding, which has another argument called encoding. Inside the protected method, x is then passed as an argument to the encoding call, i.e. encoding(x). Inside encoding, there is no condition to do something with x in case x is None. This leads to the following AttributeError when encoding(x) is executed: AttributeError: 'NoneType' object has no attribute 'shape'.

See #149