git_trace2_event_log: Fix index out of range on empty config values

In git_trace2_event_log_base.py's GetDataEventName method, it parses
value to identify if it represents a JSON list. When a config key has an
empty string value, GetDataEventName evaluates value[0], which raises
IndexError: string index out of range.

This change fixes the crash by checking if the value is a string and using
startswith/endswith to check for JSON lists instead of direct indexing.

Test: PYTHONPATH=. pytest tests/test_git_trace2_event_log.py

Bug: 512518342
TAG=agy
CONV=ff5d70d7-e5b3-42b3-8f16-23b9e3070754

Change-Id: Ic40a8c6a22df57d0e97f268f6e1bc8a14a5024a4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/602201
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Rahul Yadav <yadavrah@google.com>
Commit-Queue: Rahul Yadav <yadavrah@google.com>

tests/test_git_trace2_event_log.py

@@ -315,6 +315,7 @@ def test_data_event_config(event_log: git_trace2_event_log.EventLog) -> None:
         "repo.partialclone": "false",
         "repo.syncstate.superproject.hassuperprojecttag": "true",
         "repo.syncstate.superproject.sys.argv": ["--", "sync", "protobuf"],
+        "repo.syncstate.emptykey": "",
     }
     prefix_value = "prefix"
     event_log.LogDataConfigEvents(config, prefix_value)
@@ -323,7 +324,7 @@ def test_data_event_config(event_log: git_trace2_event_log.EventLog) -> None:
         log_path = event_log.Write(path=tempdir)
         log_data = read_log(log_path)
 
-    assert len(log_data) == 5
+    assert len(log_data) == 6
     data_events = log_data[1:]
     verify_common_keys(log_data[0], expected_event_name="version")