nuttx: if STACK_GUARD_SIZE is not set, leave it to config.h

cf. apache/nuttx-apps#2241

Author: yamt

product-mini/platforms/nuttx/wamr.mk

@@ -187,9 +187,7 @@ CSRCS += utils.c
 VPATH += $(IWASM_ROOT)/libraries/debug-engine
 endif
 
-ifeq ($(CONFIG_INTERPRETERS_WAMR_STACK_GUARD_SIZE),)
-CFLAGS += -DWASM_STACK_GUARD_SIZE=0
-else
+ifneq ($(CONFIG_INTERPRETERS_WAMR_STACK_GUARD_SIZE),)
 CFLAGS += -DWASM_STACK_GUARD_SIZE=CONFIG_INTERPRETERS_WAMR_STACK_GUARD_SIZE
 endif