Fix unspecified fallback

[lionel-]

Branched from #1111.

This adds support for combining unspecified chunks to the base::c() fallback. We can't rely on c() for this because implementations might not handle unspecified inputs, and if unspecified is the first input this would prevent dispatch from reaching the expected method. Instead, we first combine non-unspecified chunks, then use [ with NA integer locations to expand the result with missing values.

With this change, the fallback is now consistent when disjoint data frames are binded:

x <- sf::st_sfc(sf::st_point(1:2))
y <- sf::st_sfc(sf::st_multipoint(matrix(1:4, 2)), sf::st_multipoint(matrix(5:8, 2)))

vec_rbind(
  data_frame(x = x),
  data_frame(y = y)
)
#>             x                         y
#> 1 POINT (1 2)          MULTIPOINT EMPTY
#> 2 POINT EMPTY MULTIPOINT ((1 3), (2 4))
#> 3 POINT EMPTY MULTIPOINT ((5 7), (6 8))

[DavisVaughan]

Like in #1111, I assume vec-unchop support will come later?

[lionel-]

Like in #1111, I assume vec-unchop support will come later?

Yes, this might be a bit tricky with the indices.