make will_make_matrix() behave consistently for 0-length factors

[simonpcouch]

Closes #883, more context there!🪲

With CRAN version:

parsnip:::will_make_matrix(factor("a"))
#> [1] FALSE
parsnip:::will_make_matrix(factor(character()))
#> [1] TRUE

With this PR:

parsnip:::will_make_matrix(factor("a"))
#> [1] FALSE
parsnip:::will_make_matrix(factor(character()))
#> [1] FALSE

^{Created on 2023-02-23 with reprex v2.0.2}

[github-actions]

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