Error out on nil returns from MarshalTOML/MarshalText

encoding/json encodes it as "", but IMHO that's confusing, and since it
would generate invalid TOML before (`keyname =`) it's okay to change
this to an error.

Another alternative might be to skip the field if the return value is
nil, but that's probably also a bit confusing in some cases, and we can
change error to skip in the future and remain compatible, but we can't
do the reverse.

encode.go

@@ -213,13 +213,19 @@ func (enc *Encoder) eElement(rv reflect.Value) {
 		if err != nil {
 			encPanic(err)
 		}
+		if s == nil {
+			encPanic(errors.New("MarshalTOML returned nil and no error"))
+		}
 		enc.w.Write(s)
 		return
 	case encoding.TextMarshaler:
 		s, err := v.MarshalText()
 		if err != nil {
 			encPanic(err)
 		}
+		if s == nil {
+			encPanic(errors.New("MarshalText returned nil and no error"))
+		}
 		enc.writeQuoted(string(s))
 		return
 	case time.Duration:

encode_test.go

@@ -456,6 +456,44 @@ Fun = "why would you do this?"
 	}
 }
 
+type (
+	retNil1 string
+	retNil2 string
+)
+
+func (r retNil1) MarshalText() ([]byte, error) { return nil, nil }
+func (r retNil2) MarshalTOML() ([]byte, error) { return nil, nil }
+
+func TestEncodeEmpty(t *testing.T) {
+	t.Run("text", func(t *testing.T) {
+		var (
+			s   struct{ Text retNil1 }
+			buf bytes.Buffer
+		)
+		err := NewEncoder(&buf).Encode(s)
+		if err == nil {
+			t.Fatalf("no error, but expected an error; output:\n%s", buf.String())
+		}
+		if buf.String() != "" {
+			t.Error("\n" + buf.String())
+		}
+	})
+
+	t.Run("toml", func(t *testing.T) {
+		var (
+			s   struct{ Text retNil2 }
+			buf bytes.Buffer
+		)
+		err := NewEncoder(&buf).Encode(s)
+		if err == nil {
+			t.Fatalf("no error, but expected an error; output:\n%s", buf.String())
+		}
+		if buf.String() != "" {
+			t.Error("\n" + buf.String())
+		}
+	})
+}
+
 // Would previously fail on 32bit architectures; can test with:
 //   GOARCH=386         go test -c &&  ./toml.test
 //   GOARCH=arm GOARM=7 go test -c && qemu-arm ./toml.test