`cb_create()` fails with metadata without values labels

[ccsarapas]

Per docs, metadata fields for variable label and value labels can be omitted by setting to NULL. This works for omitting variable labels, but not value labels.

meta_no_val_labs <- data.frame(name = "am", label = "Transmission")
meta_no_var_lab <- data.frame(name = "mpg", val_labels = "0 = automatic; 1 = manual")

cb_create(
  mtcars,
  metadata = meta_no_var_lab,
  .var_label = NULL,
  .val_labs_sep1 = " = ", .val_labs_sep2 = "; "
)
# # A tibble: 11 × 4
#    name  type        values                    missing
#    <chr> <chr>       <chr>                       <dbl>
#  1 mpg   categorical [0] automatic; [1] manual       0
#  2 cyl   numeric     NA                              0
#  3 disp  numeric     NA                              0
#  4 hp    numeric     NA                              0
#  5 drat  numeric     NA                              0
#  6 wt    numeric     NA                              0
#  7 qsec  numeric     NA                              0
#  8 vs    numeric     NA                              0
#  9 am    numeric     NA                              0
# 10 gear  numeric     NA                              0
# 11 carb  numeric     NA                              0

cb_create(
  mtcars,
  metadata = meta_no_val_labs,
  .val_labels = NULL,
)
# Error in names(object) <- nm : attempt to set an attribute on NULL
# In addition: Warning message:
# Unknown or uninitialised column: `values`.