Handling of NA

[mgirlich]

.na argument, default error:

• jq_do()
• json_keys()

return NA of corresponding type

• jsonr_has_key*(), jsonr_has_path*()

• jsonr_extract_chr() and other scalar variants

• json_build_object()

• json_set()

• json_modify()

• json_merge()

• json_delete_path()

not yet decided:

• json_extract_lst/...(): default list()? default NULL? but what about length? or error?
• json_extract(): default from ptype? but what about length? or error?