address uninformative error for NULL engine in `nearest_neighbor()`

[simonpcouch]

parsnip/tests/testthat/_snaps/nearest_neighbor.md

Lines 28 to 32 in 4562458

	Code
	nearest_neighbor() %>% set_engine(NULL)
	Condition
	Error in `if (object$engine == "liquidSVM") ...`:
	! argument is of length zero

Noticed this in the transition to snapshot testing in #1190.

[github-actions]

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