[lit/Win] Check if a path was found before attempting to use it.

[dlj]

Summary

This appears to break some bots, when getToolsPath fails to find some or
all of the tools (for example, an incomplete GnuWin32 installation).

utils/lit/lit/llvm/config.py

def __init__(self, lit_config, config):
         if sys.platform == 'win32':
             # For tests that require Windows to run.
             features.add('system-windows')
 
             # Seek sane tools in directories and set to $PATH.
             path = self.lit_config.getToolsPath(config.lit_tools_dir,
                                            config.environment['PATH'],
                                            ['cmp.exe', 'grep.exe', 'sed.exe'])
+            if path is not None:
                 self.with_environment('PATH', path, append_path=True)
             self.use_lit_shell = True
 
         # Choose between lit's internal shell pipeline runner and a real shell.  If
         # LIT_USE_INTERNAL_SHELL is in the environment, we use that as an override.
         lit_shell_env = os.environ.get("LIT_USE_INTERNAL_SHELL")
         if lit_shell_env:
             self.use_lit_shell = lit.util.pythonize_bool(lit_shell_env)