Don't return an error when optimizing if the current parameters are invalid

[user1823]

https://forums.ankiweb.net/t/fsrs-5-1d-scheduling-and-learning-steps/50242/73

Currently, if the parameters in the FSRS parameters field are invalid, Anki returns the following error on clicking Optimize.

Error: 500: The provided FSRS parameters are invalid. Leave them blank to use the default parameters.

In my opinion, Anki should simply replace the invalid parameters with the newly optimized ones and also skip RMSE comparison that is usually done to compare old and new parameters (because RMSE can't be calculated for the invalid parameters).

@L-M-Sherlock