plugin/file: document wrong behavior in lookup fox Apex (#4376)

An apex only zone returns the wrong answer, document this by adding a
test.

Closes: #4374
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/apex_test.go

+package file
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+)
+
+const exampleApexOnly = `$ORIGIN example.com.
+@   IN      SOA     ns1.example.com. admin.example.com.  (
+                               2005011437 ; Serial
+                               1200       ; Refresh
+                               144        ; Retry
+                               1814400    ; Expire
+                               2h )       ; Minimum
+@           IN  NS      ns1.example.com.
+`
+
+func TestLookupApex(t *testing.T) {
+	// this tests a zone with *only* an apex. The behavior here is wrong, we should return NODATA, but we do a NXDOMAIN.
+	// Adding this test to document this. Note a zone that doesn't have any data is pretty useless anyway, so rather than
+	// fix this with an entirely new branch in lookup.go, just live with it.
+	zone, err := Parse(strings.NewReader(exampleApexOnly), "example.com.", "stdin", 0)
+	if err != nil {
+		t.Fatalf("Expected no error when reading zone, got %q", err)
+	}
+	fm := File{Next: test.ErrorHandler(), Zones: Zones{Z: map[string]*Zone{"example.com.": zone}, Names: []string{"example.com."}}}
+	ctx := context.TODO()
+
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+	if _, err := fm.ServeDNS(ctx, rec, m); err != nil {
+		t.Errorf("Expected no error, got %v", err)
+	}
+	if rec.Msg.Rcode != dns.RcodeNameError { // Should be RcodeSuccess in a perfect world.
+		t.Errorf("Expected rcode %d, got %d", dns.RcodeNameError, rec.Msg.Rcode)
+	}
+}