Fix ubuntu 16 unbound LD_LIBRARY_PATH error

Author: yossizap

.github/workflows/ccpp.yml

@@ -88,7 +88,7 @@ jobs:
           scripts/fetch_deps.sh
           source cutter-deps/env.sh
           export PKG_CONFIG_PATH="$CUTTER_DEPS_PYTHON_PREFIX/lib/pkgconfig:${PKG_CONFIG_PATH:-}"
-          export LD_LIBRARY_PATH="`llvm-config --libdir`:$LD_LIBRARY_PATH"
+          export LD_LIBRARY_PATH="`llvm-config --libdir`:${LD_LIBRARY_PATH:+:LD_LIBRARY_PATH:}"
         fi
         set -euo pipefail #TODO: move to top once cutter-deps doesn't fail
         if [[ "${{ matrix.cc-override }}" != "default" ]]