[Unit Test] Increase unit test coverage in sdk-actors

[mukundansundar]

Describe the proposal

Increase unit test coverage in sdk-actors

For the classes:

• runtime.ActorObjectSerializer (two public methods untested)
• runtime.ActorRuntime (around 53% line coverage) (Certain illegal arguments, exception scenarios)
• client.ActorProxyImpl (Illegal arguments, exception scenarios)

Part of dapr/dapr#2226

[manduinca]

/assign