pw_unit_test: Fix possible nullptr dereference

Change-Id: I7056e52e9926d1a90a26c210c38c9a8753cf805b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/31621
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Prashanth Swaminathan <prashanthsw@google.com>
diff --git a/pw_unit_test/framework.cc b/pw_unit_test/framework.cc
index 5b7975f..69529bc 100644
--- a/pw_unit_test/framework.cc
+++ b/pw_unit_test/framework.cc
@@ -56,7 +56,7 @@
   for (const TestInfo* test = tests_; test != nullptr; test = test->next()) {
     if (test->enabled()) {
       test->run();
-    } else {
+    } else if (event_handler_ != nullptr) {
       event_handler_->TestCaseDisabled(test->test_case());
     }
   }