fix(plugin): nilness findings (#7556)

Enable nilness linter in govet.

Plugin-by-plugin rationale:

- plugin/transfer: reuse error instead of shadowing it inside the for
  loop by declaring "ret" outside of the loop
- plugin/view: remove redundant err check
- plugin/dnstap: avoid possible nil dereference in error reporting
  path in setup test
- plugin/forward: prevent nil deference or empty-slice dereference on
  error paths in setup test

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/dnstap/setup_test.go

@@ -111,7 +111,7 @@ func TestMultiDnstap(t *testing.T) {
 	handlers := dnsserver.GetConfig(c).Handlers()
 	d1, ok := handlers[0].(*Dnstap)
 	if !ok {
-		t.Fatalf("expected first plugin to be Dnstap, got %v", reflect.TypeOf(d1.Next))
+		t.Fatalf("expected first plugin to be Dnstap, got %v", reflect.TypeOf(handlers[0]))
 	}
 
 	if d1.io.(*dio).endpoint != "dnstap1.sock" {