Fix UML build

If there aren't any boot dex files, we can't generate a boot image
profile.

Test: lunch uml-eng; m
Test: diff build-aosp_arm.ninja
Change-Id: I09cc1ef37379d9d353958c8aa75731f3d29df759
diff --git a/core/dex_preopt_libart.mk b/core/dex_preopt_libart.mk
index 64ad200..b64155c 100644
--- a/core/dex_preopt_libart.mk
+++ b/core/dex_preopt_libart.mk
@@ -100,6 +100,9 @@
 my_use_profile_for_boot_image := true
 endif
 endif
+ifeq (,$(strip $(LIBART_TARGET_BOOT_DEX_FILES)))
+my_use_profile_for_boot_image := false
+endif
 
 ifeq (true,$(my_use_profile_for_boot_image))