fix_json_null() replaces 'null' values in the JSON with an NA. This
is needed for single row tables in the JSON with a null value. In this case,
read_json(..., simplifyVector = FALSE) returns a list instead
of a data.frame because of the 'null'. This function takes that list, replaces the 'null'
with an NA and returns the data.frame.
fix_json_null(json_object)A corrected tibble with NA in place of the null.