Instead of just using {"captures":[]}.
sub functions are use captures for replacement expressions.
If we don't populate, captures for empty matches, the replacement
expression is run with an empty object as input instead of an object
containing the named captures with "" as value:
* before:
$ jq -n '"123foo456bar" | gsub("[^a-z]*(?<x>[a-z]*)"; "Z\(.x)")'
"ZfooZbarZnull"
* after:
$ jq -n '"123foo456bar" | gsub("[^a-z]*(?<x>[a-z]*)"; "Z\(.x)")'
"ZfooZbarZ"
---
I also removed a redundant
result = NULL;
if (result) {
...
}