[UpdateTestChecks] Error if --llvm-mca-binary gets an empty string

If the command line was mistyped like:
./update_mca_test_checks.py --llvm-mca-binary= /path/to/llvm-mca *.s
                                              ^-- extra whitespace

then /path/to/llvm-mca would get treated by argparse as a test-path
pattern and could actually be opened in write mode and overwritten.

llvm-svn: 334029