autoninja: prevent null pointer exception on _get_remoteexec_defaults returning None.

Change-Id: Ic89607b4e234b66253923383bc8b969c1434e9f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6693922
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Alex Ovsienko <ovsienko@google.com>

autoninja.py

@@ -355,9 +355,8 @@ def _main_inner(input_args, build_id, should_collect_logs=False):
                       file=sys.stderr)
                 use_siso = False
 
-        if use_reclient is None:
-            if use_remoteexec:
-                values = _get_remoteexec_defaults()
+        if use_reclient is None and use_remoteexec:
+            if values := _get_remoteexec_defaults():
                 if use_siso:
                     use_reclient = values["use_reclient_on_siso"]
                 else: