ctest_memcheck: Do not open empty BC output file name

In cmCTestMemCheckHandler::PostProcessBoundsCheckerTest return early
if the output file name is empty.  We already do this in the similar
cmCTestMemCheckHandler::AppendMemTesterOutput method.
Merge topic 'ctest_memcheck-no-empty-BC-output'

1c812979 ctest_memcheck: Do not open empty BC output file name