[iOS] Fix WPR test runner command empty error.

A recent change made |get_launch_command| return an empty dict, which
caused a none-empty check failure later. This change set some default
fields in the dict as what was happening before.

Bug: 1054989
Change-Id: Ib2cfeac297705eb4e20c40b923c30d20976b6852
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083761
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746593}

ios/build/bots/scripts/wpr_runner.py

@@ -392,9 +392,10 @@ class WprProxySimulatorTestRunner(test_runner.SimulatorTestRunner):
     """
 
     test_config = {}
+    test_config['invert'] = False
+    test_config['test_filter'] = []
     if test_app:
       if test_app.included_tests:
-        test_config['invert'] = False
         test_config['test_filter'] = test_app.included_tests
       elif test_app.excluded_tests:
         test_config['invert'] = True