Update dmarc.lua

Update dmarc.lua

When from.name is Nil its still show up, with != "" not

Then you get
From: via user <user@domain.tld>

Instead of
From: otheruser via user <user@domain.tld>

Debug:
2023-10-12 12:08:15 #725504(normal) <0b948d>; dmarc; dmarc.lua:191: munging debug: {[user] = user, [domain] = domain.tld, [flags] = {[valid] = true}, [name] = , [raw] = user@domain.tld, [addr] = user@domain.tld}