Fix build error from llndk.libraries.txt

llndk.libraries.txt build fails from clang tools build, as there is no
valid target path for llndk.libraries.txt from the build. This change
skips llndk.libraries.txt build if target output path is empty.

Bug: 330100430
Test: prebuilts/clang-tools/build-prebuilts.sh passed
Change-Id: I867dc90dbf98e3d8c24b191a744686286740b230