Improvement in Client.newRpcStub(...) : Constructor may not allow NULL values

[longquanzheng]

In the current implementation, in order to get a "Mock" of the workflow class, we use the first constructor, and pass all null values into the constructor to create he mock instance.

iwf-java-sdk/src/main/java/io/iworkflow/core/Client.java

Line 676 in 71172ee

final Object[] params = new Object[parameterCount];

However, this may be a problem if the constructor doesn't allow NULL values.
See the example of broken experience here: #260

This is usually not a problem because usually the iwf workflow only need to take in a list of state or its dependency.

An idea is to now rely on the real constructor -- i there is a way to create the instance bypassing the real class constructor