core: don't add file deps for modules without any installed files

Change-Id: I0acadb2ce287a1657d29dc787bd9b1c2f95aa918
Signed-off-by: Dima Zavin <dima@android.com>
diff --git a/core/main.mk b/core/main.mk
index 113bf4b..9a566ca 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -569,11 +569,13 @@
   $(eval r := $(ALL_MODULES.$(m).REQUIRED)) \
   $(if $(r), \
     $(eval r := $(call module-installed-files,$(r))) \
-    $(eval $(call add-required-deps,$(ALL_MODULES.$(m).INSTALLED),$(r))) \
+    $(eval i := $(ALL_MODULES.$(m).INSTALLED)) \
+    $(eval $(if $(i), $(call add-required-deps,$(i),$(r)))) \
    ) \
  )
 m :=
 r :=
+i :=
 add-required-deps :=
 
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