Do not encode null values to JSON for Semtech UDP backend.

The Semtech UDP Packet Forwarder interprets NULL as 0 (at least in the
case of `tmst`), which can cause issues in some scenarios, e.g.
https://support.chirpstack.io/questions/D1H4/gateway-s-rejects-class-b-package-s-packet-rejected-timestamp-seems-wrong-too-much-in-advance

Author: brocaar

src/backend/semtech_udp/structs.rs

@@ -783,8 +783,10 @@ pub struct TxPk {
     /// Concentrator board used for RX (unsigned integer).
     pub brd: u8,
     /// Send packet on a certain timestamp value (will ignore time).
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub tmst: Option<u32>,
     /// Send packet at a certain GPS time (GPS synchronization required).
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub tmms: Option<u64>,
     /// TX central frequency in MHz (unsigned float, Hz precision).
     pub freq: f64,
@@ -793,14 +795,19 @@ pub struct TxPk {
     /// LoRa datarate identifier (eg. SF12BW500) || FSK datarate (unsigned, in bits per second).
     pub datr: DataRate,
     /// LoRa ECC coding rate identifier.
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub codr: Option<CodeRate>,
     /// FSK frequency deviation (unsigned integer, in Hz).
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub fdev: Option<u16>,
     /// If true, disable the CRC of the physical layer (optional).
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub ncrc: Option<bool>,
     /// Lora modulation polarization inversion.
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub ipol: Option<bool>,
     /// RF preamble size (unsigned integer).
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub prea: Option<u16>,
     /// RF packet payload size in bytes (unsigned integer).
     pub size: u16,

tests/semtech_udp_test.rs

@@ -226,7 +226,7 @@ async fn end_to_end() {
     let size = socket.recv(&mut buffer).await.unwrap();
     assert_eq!(&[2, 210, 4, 3], &buffer[..4]);
     let json = String::from_utf8_lossy(&buffer[4..size]);
-    assert_eq!("{\"txpk\":{\"imme\":false,\"rfch\":0,\"powe\":16,\"ant\":0,\"brd\":0,\"tmst\":1001234,\"tmms\":null,\"freq\":868.3,\"modu\":\"LORA\",\"datr\":\"SF8BW125\",\"codr\":\"4/5\",\"fdev\":null,\"ncrc\":null,\"ipol\":false,\"prea\":null,\"size\":3,\"data\":\"AQID\"}}", json);
+    assert_eq!("{\"txpk\":{\"imme\":false,\"rfch\":0,\"powe\":16,\"ant\":0,\"brd\":0,\"tmst\":1001234,\"freq\":868.3,\"modu\":\"LORA\",\"datr\":\"SF8BW125\",\"codr\":\"4/5\",\"ipol\":false,\"size\":3,\"data\":\"AQID\"}}", json);
 
     // TX_ACK
     socket