na.rm = TRUE not respected in pmin/pmax for Snowflake

[fh-mthomson]

As discussed in #118, the default translations for pmin() and pmax() are LEAST and GREATEST, respectively.

However, for Snowflake (akin to Oracle), LEAST and GREATEST do the following (in contrast to many other backends, discussed with Snowflake here and a community-identified workaround here):

If any of the argument values is NULL, the result is NULL.

Accordingly:

• when providing na.rm = TRUE to pmin() and pmax(), it's silently ignored currently (related to dbplyr translation of pmax/pmin throws an error if it receives the na.rm = TRUE argument #479)
• The default, periodic messaging for na.rm = FALSE in check_na_rm()) is not accurate.

I'd expect either:

1. [preferred] na.rm = TRUE is supported OR
2. [interim] na.rm = TRUE errors when called on Snowflake (and does not warn when na.rm = FALSE)

Illustrative Code (masking company Snowflake instance):

conn_snowflake <- DBI::dbConnect(odbc::odbc(), .connection_string = "<company>")
id_temp <- DBI::Id(schema = "TEST", table = "TEST")

test_tbl <- tibble::tibble(
  a = c(1, NA, 3),
  b = c(NA, 2, 4)
)

dbplyr::copy_to(
  conn_snowflake,
  test_tbl,
  name = id_temp,
  overwrite = TRUE
)

tbl_sf <- conn_snowflake %>%
  tbl(id_temp)

# expected behavior: correctly ignores NA for `min()` / `max()`
tbl_sf %>%
  dplyr::summarise(
    min_a = min(a, na.rm = TRUE),
    min_b = min(b, na.rm = TRUE),
    max_a = max(a, na.rm = TRUE),
    max_b = max(b, na.rm = TRUE)
  )

# Source:   SQL [1 x 2]
# Database: Snowflake 7.22.1[michael.thomson@Snowflake/TEST]
  min_a min_b max_a max_b
  <dbl> <dbl> <dbl> <dbl>
1     1     2     3     4

na_rm <- TRUE # toggle TRUE or FALSE

# unexpected behavior: both return NA for each na.rm = TRUE and FALSE
tbl_sf %>%
  dplyr::mutate(
    pmin_a_b = pmin(a, b, na.rm = na_rm),
    pmax_a_b = pmax(a, b, na.rm = na_rm)
  )

# Source:   SQL [3 x 4]
# Database: Snowflake 7.22.1[michael.thomson@Snowflake/TEST]
      a     b pmin_a_b pmax_a_b
  <dbl> <dbl>    <dbl>    <dbl>
1     1    NA       NA       NA
2     3     4        3        4
3    NA     2       NA       NA