multi_dots doesn't re-calculate n lengths after exploding

[Kevanness]

Ran into an issue when using req_url_query(.multi = "explode") with a mix of multi-length and NULL params. It looks like n isn't re-calculated after exploding in multi_dots() on L66 which causes n to be shorter than dots creating a select/recycling issue.

expect_equal(
  multi_dots(a = 1:2, b = NULL, .multi = "explode"),
  list(a = I("1"), a = I("2"), b = NULL)
)

expect_equal(
  multi_dots(a = 1:3, b = NULL, .multi = "explode"),
  list(a = I("1"), a = I("2"), a = I("3"), b = NULL)
)

Since it's using explode, then I would think that n would always have a value of 0 or 1 removing the need of the n > 1 handling for explode. I think it could just have the following for the .multi == "explode" section?

dots <- explode(dots)
n <- lengths(dots)