Allow source_params=None in LikelihoodLISASMBH_LinearResiduals constructor

According to docstring in LikelihoodLISASMBH_LinearResiduals which is located in lisabeta/lisa/lisa.py, the argument source_params is ignored when signal is in input data.

Nevertheless, the current implementation does not allow setting it to None even when data is provided and data_noise_only is False. source_params is always assumed source parameter dictionary and required during initialisation no matter whether it is going to be ignored. Therefore, the user is obliged to provide a source_params dictionary even if they did not mean to use it.

@marsat Is it possible to allow source_params=None in the implementation?