[ios] Fix crash in CreateSyntheticCrashReportForUte

CreateSyntheticCrashReportForUte can be legitimately called with empty
breadcrumbs vector, in which case the app should not crash.

This CL uses "No Breadcrumbs" signature instead of trying to access the
element in an empty vector.

Bug: 1154039
Change-Id: I45537944122700c9085c4e824ae9edeafe1c7395
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566361
Commit-Queue: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832199}

ios/chrome/browser/crash_report/synthetic_crash_report_util.mm

@@ -106,8 +106,11 @@ void CreateSyntheticCrashReportForUte(
                                       base::SysInfo::HardwareModelName());
   AppendConfigWithBreakpadServerParam(config, "breadcrumbs",
                                       base::JoinString(breadcrumbs, "\n"));
-  AppendConfigWithBreakpadServerParam(
-      config, "signature", breadcrumbs.back().substr(strlen("00:00 ")));
+
+  std::string signature = breadcrumbs.empty()
+                              ? "No Breadcrumbs"
+                              : breadcrumbs.back().substr(strlen("00:00 "));
+  AppendConfigWithBreakpadServerParam(config, "signature", signature);
 
   for (NSString* key in previous_session.reportParameters.allKeys) {
     AppendConfigWithBreakpadServerParam(