Fix build error on empty ramdisk

package `init_vendor` is the only content of ramdisk so far.
We would get build error if we do not include init_vendor.
The patch fix the build error for the case that ramdisk is empty.

Bug: 129386309
Test: lunch aosp_arm64_ab-userdebug; make ramdisk -j
Test: Build pass
Change-Id: I7c7c828b5f29350268d4789393b90740dd68162d

core/Makefile

@@ -894,6 +894,7 @@ INSTALLED_FILES_JSON_RAMDISK := $(INSTALLED_FILES_FILE_RAMDISK:.txt=.json)
 $(INSTALLED_FILES_FILE_RAMDISK): .KATI_IMPLICIT_OUTPUTS := $(INSTALLED_FILES_JSON_RAMDISK)
 $(INSTALLED_FILES_FILE_RAMDISK) : $(INTERNAL_RAMDISK_FILES) $(FILESLIST)
 	@echo Installed file list: $@
+	@mkdir -p $(TARGET_RAMDISK_OUT)
 	@mkdir -p $(dir $@)
 	@rm -f $@
 	$(hide) $(FILESLIST) $(TARGET_RAMDISK_OUT) > $(@:.txt=.json)