plugin/file: New zone should have zero records (#3025)

After calling NewZone the number of records should be zero, but due to
how zone.All() was implemented so empty RRs would be added. This then
fails the == 0 check in xfr.go and put nil in the slice, this then
subsequently panics on the Len().

Fix this making All() smarter when adding records. Added little test to
enfore this.
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/xfr_test.go

@@ -3,6 +3,7 @@ package file
 import (
 	"fmt"
 	"strings"
+	"testing"
 )
 
 func ExampleZone_All() {
@@ -32,3 +33,11 @@ func ExampleZone_All() {
 	// xfr_test.go:15: a.miek.nl.	1800	IN	A	139.162.196.78
 	// xfr_test.go:15: a.miek.nl.	1800	IN	AAAA	2a01:7e00::f03c:91ff:fef1:6735
 }
+
+func TestAllNewZone(t *testing.T) {
+	zone := NewZone("example.org.", "stdin")
+	records := zone.All()
+	if len(records) != 0 {
+		t.Errorf("Expected %d records in empty zone, got %d", 0, len(records))
+	}
+}

plugin/file/zone.go

@@ -173,15 +173,21 @@ func (z *Zone) All() []dns.RR {
 		records = append(records, a.All()...)
 	}
 
+	// Either the entire Apex is filled or none it, this isn't enforced here though.
 	if len(z.Apex.SIGNS) > 0 {
 		records = append(z.Apex.SIGNS, records...)
 	}
-	records = append(z.Apex.NS, records...)
+	if len(z.Apex.NS) > 0 {
+		records = append(z.Apex.NS, records...)
+	}
 
 	if len(z.Apex.SIGSOA) > 0 {
 		records = append(z.Apex.SIGSOA, records...)
 	}
-	return append([]dns.RR{z.Apex.SOA}, records...)
+	if z.Apex.SOA != nil {
+		return append([]dns.RR{z.Apex.SOA}, records...)
+	}
+	return records
 }
 
 // NameFromRight returns the labels from the right, staring with the