Don't set empty fields in output

[rmilecki]

Before introducing parsers concept extract() did not set empty fields in
output. So in case of re.findall() returning an empty list there was a
warning like:
WARNING:invoice2data.extract.invoice_template:regexp for field <foo> didn't match
and on <foo> in output dictionary / file.

That has unintentionally changed after commit 670f8729b7b6 ("Extend
fields syntax to support associative arrays & add 2 initial parsers
(#308)"). Output can now look like:
[
    {
        "issuer": "Company",
        "amount": 12.34,
        "date": "2023-01-01",
        "invoice_number": "01/2023",
        "test_field": [],
        "currency": "EUR",
        "desc": "Invoice from Company"
    }
]

Bring back old behaviour and don't set empty fields:
[
    {
        "issuer": "Company",
        "amount": 12.34,
        "date": "2023-01-01",
        "invoice_number": "01/2023",
        "currency": "EUR",
        "desc": "Invoice from Company"
    }
]

[rmilecki]

I've just noticed that my old #308 changed parsing behaviour.

I'm wondering if the old behaviour made more sense and should be brought back. Any opinion on this?

[bosd]

I am kinda neutral on this. Both methods has his up and downsides.
But I tend to lean towards returning to the old behavior.

[bosd]

Let's restore the old behavior. :+1