Missing values in key_cols under rlistings::as_listing()

[qiliu1013]

Not Sure if these have been resolved, below are the improvement you may consider.

• rlistings::as_listing() cannot handle if there were missing values in the variables that listed in key_col. The error message is not readable.
  Error in h(simpleError(msg, call)) : error in evaluating the argument 'x' in selecting a method for function 'toString': values must be length 1, but FUN(X[[1]]) result is length 0

• fmt_config() doesn't work for character. Related to above issue, it doesn't work if na_str = "<No data>" was needed in charater variables that required in the listings.
  `

packageVersion("rlistings")
[1] ‘0.2.4’
`

Hi @shajoezhu, above are the issues we found when we test TLGc - PKCL01. It can be resolved if we do some data pre-processing, but the error message is confusing if we don't have this background info.