Avoid spurios SPI error with 0-length transactions

main/spi_leds.cpp

@@ -107,6 +107,9 @@ static uint8_t* encode_byte(uint8_t *out, uint8_t in) {
 }
 
 void SPI_LEDs::show() {
+    // We generate a spurious SPI error with a 0-length transaction.
+    if (length <= 0) return;
+
     static int index = 0;
     int ret;
     const Color *pixel = pixels;