FIX: avoid usage of dig when looking for job class (#17772)

`{a: "a"}.dig(:a, :b)` will result in an exception, since ruby assumes that `"a"` will be another hash it can look up the `:b` key on.

lib/discourse.rb

@@ -175,9 +175,13 @@ module Discourse
     context ||= {}
     parent_logger ||= Sidekiq
 
-    job = context.dig(:job, "class")
-    if job
-      job_exception_stats[job] += 1
+    job = context[:job]
+
+    if Hash === job
+      job_class = job["class"]
+      if job_class
+        job_exception_stats[job_class] += 1
+      end
     end
 
     cm = RailsMultisite::ConnectionManagement