Allow ini(model) <- NULL to drop the full ini block

[billdenney]

Related to #520

When doing a PD fit contingent on a PK model, I will sometimes want to drop all parameters from the PK model to move them to the dataset. What I currently do is:

ini(ret) <- str2lang("ini({})")

I think it would be simpler (and generally consistent with other R tools) to be able to do

ini(ret) <- NULL

[mattfidler]

This works only if there isn't a endpoint defined.