arm/lib/fvp/runner: don't pass '' as cwd

Don't pass "" as the cwd as that fails, use None so the cwd doesn't get
changed.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/lib/fvp/runner.py

@@ -100,7 +100,7 @@ class FVPRunner:
                 env[name] = os.environ[name]
 
         # Allow filepath to be relative to fvp configuration file
-        cwd = os.path.dirname(fvpconf)
+        cwd = os.path.dirname(fvpconf) or None
         self._logger.debug(f"FVP call will be executed in working directory: {cwd}")
 
         self._logger.debug(f"Constructed FVP call: {shlex_join(cli)}")