Handle nil sampling config

[RonFed]

An implementation of the ConfigProvider interface might provide configurations that don't specify a sampler.

• For the initial configuration, the NewSamplingManager function is used - where a default config will be used if non is present.
• For an update in configuration the applyConfig function will be used (this is not fully implemented yet in the main manager). In that case, the ConfigLoop handler will avoid calling the sampling applyConfig if no config was provided - this will make that update a no-op for sampling. For this reason, the applyConfig function is changed to return an error if a nil config is provided.