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