check variable is empty before add into dependency - makefile

Question

below code from AOSP build/core/Makefile
If someone set INSTALLED_RAMDISK_TARGET as empty(by accident) then recoveryimage will have no RAMDISK
How can I check if each of the dependencies is empty or not?
or is there any other suggestion?
1016 $(INSTALLED_RECOVERYIMAGE_TARGET): $(MKBOOTFS) $(MKBOOTIMG) $(MINIGZIP) \
1017 $(INSTALLED_RAMDISK_TARGET) \
1018 $(INSTALLED_BOOTIMAGE_TARGET) \
1019 $(INTERNAL_RECOVERYIMAGE_FILES) \
1020 $(recovery_initrc) $(recovery_sepolicy) $(recovery_kernel) \
1021 $(INSTALLED_2NDBOOTLOADER_TARGET) \
1022 $(recovery_build_prop) $(recovery_resource_deps) \
1023 $(recovery_fstab) \
1024 $(RECOVERY_INSTALL_OTA_KEYS)
1025 $(call build-recoveryimage-target, $#)
I tried below method but in vain because foreach will skip variable which is empty
1016 RECOVERYIMAGE_REQUIRED := $(MKBOOTFS) $(MKBOOTIMG) $(MINIGZIP) \
1017 $(INSTALLED_RAMDISK_TARGET) \
1018 $(INSTALLED_BOOTIMAGE_TARGET) \
1019 $(INTERNAL_RECOVERYIMAGE_FILES) \
1020 $(recovery_initrc) $(recovery_sepolicy) $(recovery_kernel) \
1021 $(INSTALLED_2NDBOOTLOADER_TARGET) \
1022 $(recovery_build_prop) $(recovery_resource_deps) \
1023 $(recovery_fstab) \
1024 $(RECOVERY_INSTALL_OTA_KEYS)
1025 $(call build-recoveryimage-target, $#)
1026 $(foreach item,$(HOSDIMAGE_REQUIRED), \
1027 $(eval _item := $(strip $(item))) \
1028 $(if $(_item),$(info $(_item) checked),$(error dependency is empty)) \
1029 )
1030 $(INSTALLED_RECOVERYIMAGE_TARGET): $(RECOVERYIMAGE_REQUIRED)

Answer 1

Oh how I wish for a --error-undefined-variables to match the existing --warn-undefined-variables. In the mean time you can just use something like:
depvarnames := \
MKBOOTFS \
MKBOOTIMG \
MINIGZIP \
⋮
deps := $(foreach _,${depvarnames},$(or $_,$(error $$$_ is empty!)))
${INSTALLED_RECOVERYIMAGE_TARGET}: ${deps}
recipe
⋮

Answer 2

below is my solution, looks stupid but it really help me a lot
NULL :=
SPACE := $(NULL) $(NULL)
DQUOTE = "
RECOVERYIMAGE_REQUIRED := \
"$(MKBOOTFS)" \
"$(MKBOOTIMG)" \
"$(MINIGZIP)" \
"$(PRODUCT_OUT)/ramdisk.img" \
"$(PRODUCT_OUT)/boot.img" \
"$(rec_initrc)" \
"$(rec_kernel)" \
"$(rec_build_prop)" \
"$(rec_resource_deps)" \
"$(rec_fstab)"
$(foreach item, $(RECOVERYIMAGE_REQUIRED), \
$(eval _item := $(strip $(item))) \
$(if $(filter "", $(_item)),$(error dependency is empty),) \
)
RECOVERYIMAGE_OPTIONAL := \
"$(INSTALLED_2NDBOOTLOADER_TARGET)" \
$(foreach item, $(RECOVERYIMAGE_REQUIRED) $(RECOVERYIMAGE_OPTIONAL), \
$(if $(strip $(subst $(DQUOTE),$(SPACE),$(item))), \
$(eval RECOVERYIMAGE_FINAL_LIST += $(strip $(subst $(DQUOTE),$(SPACE),$(item)))), \
) \
)
$(INSTALLED_RECOVERYIMAGE_TARGET): $(RECOVERYIMAGE_FINAL_LIST)