Handle empty time string RXPK (#82)

Some gateway vendors send and empty `time` string instead of leaving this `time` key` out of the JSON object, or setting this to `null`.

Author: moznion

internal/gateway/backend.go

@@ -445,7 +445,9 @@ func newRXPacketsFromRXPK(mac lorawan.EUI64, rxpk RXPK) ([]gw.RXPacketBytes, err
 
 	if rxpk.Time != nil {
 		ts := time.Time(*rxpk.Time)
-		rxPacket.RXInfo.Time = &ts
+		if !ts.IsZero() {
+			rxPacket.RXInfo.Time = &ts
+		}
 	}
 
 	if rxpk.Tmms != nil {

internal/gateway/structs.go

@@ -316,11 +316,19 @@ type CompactTime time.Time
 
 // MarshalJSON implements the json.Marshaler interface.
 func (t CompactTime) MarshalJSON() ([]byte, error) {
-	return []byte(time.Time(t).UTC().Format(`"` + time.RFC3339Nano + `"`)), nil
+	t2 := time.Time(t)
+	if t2.IsZero() {
+		return []byte("null"), nil
+	}
+	return []byte(t2.UTC().Format(`"` + time.RFC3339Nano + `"`)), nil
 }
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
 func (t *CompactTime) UnmarshalJSON(data []byte) error {
+	if string(data) == `""` {
+		return nil
+	}
+
 	t2, err := time.Parse(`"`+time.RFC3339Nano+`"`, string(data))
 	if err != nil {
 		return err

internal/gateway/structs_test.go

@@ -79,6 +79,22 @@ func TestCompactTime(t *testing.T) {
 			})
 		})
 	})
+
+	Convey("Given an empty string as date value", t, func() {
+		Convey("UnmarshalJSON returns nil", func() {
+			var ct CompactTime
+			err := ct.UnmarshalJSON([]byte(`""`))
+			So(err, ShouldBeNil)
+			So(time.Time(ct).Equal(time.Time{}), ShouldBeTrue)
+		})
+
+		Convey("MarshalJSON returns null", func() {
+			ct := CompactTime(time.Time{})
+			b, err := ct.MarshalJSON()
+			So(err, ShouldBeNil)
+			So(string(b), ShouldEqual, "null")
+		})
+	})
 }
 
 func TestGetPacketType(t *testing.T) {