Support empty selections in all recipe steps

[DavisVaughan]

This PR adds empty selection support to all recipe steps.

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We thought that the best way to do this was to return early from each step, returning an alternate step_empty_selection() step, but this was problematic for a few reasons:

• We lose the column structure when you tidy() that step in the recipe, because the empty selection step doesn't know what columns should be there
• If you prep a recipe, then reprep it with fresh = TRUE, things got a little complicated because ideally you'd reprep with the original step, rather than the empty selection step we inserted
• It isn't type stable (minor issue)
• It still feels like a bit of a hack

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SO, I've gone through all of the steps, ensuring that each one supports empty selection natively. Most required minimal changes, which is great. A few notes:

• I've replaced all usage of ellipse_check() with enquos(), since now empty selections are okay. This function is exported, so I also mentioned that it is sort of deprecated now in favor of supporting empty selections directly (but I didn't officially deprecate it).
• Each step now has 3 tests associated with it related to empty selections. One tests prep() / bake(), one tests the print method, and one tests the tidy() method before and after prepping.
• Going through all of this fixed a number of small edge case bugs that weren't worth a NEWS bullet. In particular, our tidy methods often contained columns that were named vectors. These have all now been un-named.

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