Easiest way to *not* output a dash from unset capture output

We are using a custom log capture format (JSON) and have quite a few captures in place.

What’s the easiest and most performant way to output a blank string instead of the capture string if it wasn’t set?

Specifically, we have a custom log format with this in it:

          "user-agent"  :"%[capture.req.hdr(0),json(utf8s)]",
          "referer"     :"%[capture.req.hdr(1),json(utf8s)]",
          "api-key"     :"%[capture.req.hdr(2),json(utf8s)]",

but if the respective headers weren’t set we get a dash - as the capture output.

I’d rather see just a blank field. What’s the easiest way to rewrite these captures to get a blank value instead?

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