[debugger] only serialize audit trail when enabled

Writing the audit trail name to the JSON writer, but no audit
trail was causing a failed assertion(1).

(1) https://skia.googlesource.com/skia/+/8d3e00a1f25fbbebe0dafa40fcc13ea9c32b5bec/src/utils/SkJSONWriter.h#128

Bug: skia:none
Change-Id: I4d1ce85d7064d1928db610c063f1222c7e710cb6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/725536
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Chris Mumford <cmumford@google.com>
1 file changed