Single dot values & null MX records

[bjxrn]

I've noticed this package doesn't properly parse null MX records (RFC7505).

A null MX record can be used to signal that the domain does not accept any mail. A valid null MX record would be
example.com. 86400 IN MX 0 .

In a null MX record the target is a single dot. However, in spatie/dns the validation/sanitation flow trims dots, which turns the target into an empty string, which results in a parsing error. This means you get the same result when you query a domain with no MX records as with a domain with a null MX record.

I could create a pull request that bypasses this in the MX castTarget() function, but I'm not sure if there are other types of records which also would allow dots as valid hostnames/targets, in which case the fix might have to be made in a different place.

Is "." always a valid target? Or is this something MX specific?