Fix direnv error when LD_LIBRARY_PATH is initially unset

Author: cydparser

ghc.nix

@@ -247,7 +247,7 @@ hspkgs.shellFor rec {
 
     # "nix-shell --pure" resets LANG to POSIX, this breaks "make TAGS".
     export LANG="en_US.UTF-8"
-    export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:${lib.makeLibraryPath depsSystem}"
+    export LD_LIBRARY_PATH="''${LD_LIBRARY_PATH:+''${LD_LIBRARY_PATH}:}${lib.makeLibraryPath depsSystem}"
     unset LD
 
     ${lib.optionalString withDocs "export FONTCONFIG_FILE=${fonts}"}