make find_lib_path() work if LD_LIBARY_PATH is not set

If the `LD_LIBRARY_PATH` environment variable is not set, `utils.find_lib_path()` fails, so I cannot import `scikits.cuda.cublas`. Fixed with this pull request. (Bonus: Skips the empty string that occurs if you do `export LD_LIBRARY_PATH="$LD_LIBRARY_PATH":/foo/bar:/some/more` with a previously empty `LD_LIBRARY_PATH`, which is probably a common pattern.)