Let get-prebuilt-src-arch return empty if the input is empty

The list of archs of a pure java apk (no native libs) is empty.
In this case get-prebuilt-src-arch still return 'none'.
If it's set to LOCAL_MODULE_TARGET_ARCH, it makes the apk
be incompatible with the building target. It's not good.

Just return empty in this case.

Change-Id: Ide75cdb08c8480c8e7d3a0ac59f41831fbc18bd5
diff --git a/core/definitions.mk b/core/definitions.mk
index 5e45e1f..edcc1b2 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2515,12 +2515,13 @@
 endef
 
 # Return the arch for the source file of a prebuilt
-# Return "none" if no matching arch found, so the result can be passed to
+# Return "none" if no matching arch found and return empty
+# if the input is empty, so the result can be passed to
 # LOCAL_MODULE_TARGET_ARCH.
 # $(1) the list of archs supported by the prebuilt
 define get-prebuilt-src-arch
 $(strip $(if $(filter $(TARGET_ARCH),$(1)),$(TARGET_ARCH),\
-  $(if $(filter $(TARGET_2ND_ARCH),$(1)),$(TARGET_2ND_ARCH),none)))
+  $(if $(filter $(TARGET_2ND_ARCH),$(1)),$(TARGET_2ND_ARCH),$(if $(1),none))))
 endef
 
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