Fix chip_variant retrieval with default value

Author: Jason2866

tools/pioarduino-build.py

@@ -32,7 +32,7 @@
 platform = env.PioPlatform()
 board_config = env.BoardConfig()
 build_mcu = board_config.get("build.mcu", "").lower()
-chip_variant = board_config.get("build.chip_variant").lower()
+chip_variant = board_config.get("build.chip_variant", "").lower()
 chip_variant = chip_variant if chip_variant else build_mcu
 partitions_name = board_config.get("build.partitions", board_config.get("build.arduino.partitions", ""))