Only update the serial number if it hasn't already been set

This fixes the serial number for Nintendo Switch Pro, which is queried from the hardware in device initialization, and was later clobbered by the USB string which isn't correct.

(cherry picked from commit 2042e9c4e3cba6ffa9c34abac14828e31365f98b)

src/joystick/hidapi/SDL_hidapijoystick.c

@@ -676,6 +676,22 @@ static void HIDAPI_UpdateJoystickSerial(SDL_HIDAPI_Device *device)
     }
 }
 
+static SDL_bool HIDAPI_SerialIsEmpty(SDL_HIDAPI_Device *device)
+{
+    SDL_bool all_zeroes = SDL_TRUE;
+
+    if (device->serial) {
+        const char *serial = device->serial;
+        for (serial = device->serial; *serial; ++serial) {
+            if (*serial != '0') {
+                all_zeroes = SDL_FALSE;
+                break;
+            }
+        }
+    }
+    return all_zeroes;
+}
+
 void HIDAPI_SetDeviceSerial(SDL_HIDAPI_Device *device, const char *serial)
 {
     if (serial && *serial && (!device->serial || SDL_strcmp(serial, device->serial) != 0)) {
@@ -1079,7 +1095,9 @@ static void HIDAPI_UpdateDeviceList(void)
                     device->seen = SDL_TRUE;
 
                     /* Check to see if the serial number is available now */
-                    HIDAPI_SetDeviceSerialW(device, info->serial_number);
+                    if(HIDAPI_SerialIsEmpty(device)) {
+                        HIDAPI_SetDeviceSerialW(device, info->serial_number);
+                    }
                 } else {
                     HIDAPI_AddDevice(info, 0, NULL);
                 }