Allow NULL to remove columns

[hadley]

library(dplyr, warn.conflicts = FALSE)

dbplyr::memdb_frame(x = 1:5, y = 1:5) %>% 
  mutate(
    x1 = x + 1,
    z = x1 + x,
    x1 = NULL
  ) %>% 
  show_query()
#> Error in if (length(ans) == 0L || as.character(ans[[1L]])[1L] == "~") {: missing value where TRUE/FALSE needed

^{Created on 2018-12-24 by the reprex package (v0.2.1)}

[hadley]

Requires r-lib/rlang#715 first

[hadley]

Minimal reprex:

library(dbplyr)
translate_sql(NULL)
#> Error in if (length(ans) == 0L || as.character(ans[[1L]])[1L] == "~") {: missing value where TRUE/FALSE needed

^{Created on 2019-02-06 by the reprex package (v0.2.1.9000)}