`set_engine(object, engine)` should be `set_engine(object, engine = NULL)`

[DavisVaughan]

Since if engine = NULL we choose a default for the user with a message
https://github.com/tidymodels/parsnip/pull/488/files

It is better if this is explicit in the function signature, and documented in the help file.

Possibly also set_mode(object, mode = NULL)?

[juliasilge]

We don't currently have checking + choosing a default like this:

parsnip/R/engines.R

Line 16 in 6d6b0a7

object$engine <- avail_eng[1]

for mode because it doesn't make sense for all models, and it's tough to be able to tell at that point.

We currently throw a warning if we choose a default for engine, which feels strange to me if NULL is explicit in the function signature and what it does is documented. Should that be deescalated to a message?

[DavisVaughan]

It's odd because it looks like we tried to do something similar with mode here:

parsnip/R/arguments.R

Lines 79 to 81 in a546656

	if (rlang::is_missing(mode)) {
	mode <- NULL
	}

But then we abort on null modes here

parsnip/R/aaa_models.R

Line 146 in 6d6b0a7

rlang::abort(compatible_modes)

[github-actions]

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