Check packages/modules before packing otatools keys

Fix build error when packages/modules is not existed.

Bug: 256565230
Test: make dist, check otatools.zip
Change-Id: I4153b143e677e35f95a3d47663cb8efcde7abdbf
diff --git a/core/Makefile b/core/Makefile
index a3b79a9..913abfc 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -5045,9 +5045,11 @@
   $(sort $(shell find build/make/target/product/security -type f -name "*.x509.pem" -o \
       -name "*.pk8"))
 
+ifneq (,$(wildcard packages/modules))
 INTERNAL_OTATOOLS_PACKAGE_FILES += \
   $(sort $(shell find packages/modules -type f -name "*.x509.pem" -o -name "*.pk8" -o -name \
       "key.pem"))
+endif
 
 ifneq (,$(wildcard device))
 INTERNAL_OTATOOLS_PACKAGE_FILES += \