Don't add empty entry to PATH.

When ANDROID_TOOLCHAIN_2ND_ARCH is unset (all arches except ARM64) the
PATH was being extended with an empty entry, which is interpreted as the
current directory (rarely desirable).

Test: lunch aosp_arm-eng; echo $PATH; lunch aosp_arm64-eng; echo $PATH
Change-Id: I31661241c10b06165dcf555c8b3e799fe54cc0dc
diff --git a/envsetup.sh b/envsetup.sh
index a8bef92..9bb0d24 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -231,7 +231,12 @@
     fi
 
     export ANDROID_DEV_SCRIPTS=$T/development/scripts:$T/prebuilts/devtools/tools:$T/external/selinux/prebuilts/bin
-    export ANDROID_BUILD_PATHS=$(get_build_var ANDROID_BUILD_PATHS):$ANDROID_TOOLCHAIN:$ANDROID_TOOLCHAIN_2ND_ARCH:$ANDROID_DEV_SCRIPTS:
+    ANDROID_BUILD_PATHS=$(get_build_var ANDROID_BUILD_PATHS):$ANDROID_TOOLCHAIN
+    if [ -n "$ANDROID_TOOLCHAIN_2ND_ARCH" ]; then
+        ANDROID_BUILD_PATHS=$ANDROID_BUILD_PATHS:$ANDROID_TOOLCHAIN_2ND_ARCH
+    fi
+    ANDROID_BUILD_PATHS=$ANDROID_BUILD_PATHS:$ANDROID_DEV_SCRIPTS:
+    export ANDROID_BUILD_PATHS
 
     # If prebuilts/android-emulator/<system>/ exists, prepend it to our PATH
     # to ensure that the corresponding 'emulator' binaries are used.