[Minor] Dmarc: Fix bad policies detection

src/plugins/lua/dmarc.lua

@@ -664,7 +664,7 @@ local function dmarc_callback(task)
             end
           end
 
-          if not has_valid_policy then
+          if not has_valid_policy and not seen_invalid then
             policy_target.err = lookup_domain .. ':' .. ' no valid DMARC record'
             policy_target.symbol = dmarc_symbols['na']
           end