FIX: Ensure RequestTracker handles bubbled exceptions correctly (#26940)

This can happen for various reasons including rate limiting and middleware bugs. This should resolve the warning we're seeing in the logs

```
RequestTracker.get_data failed : NoMethodError : undefined method `[]' for nil:NilClass
```

lib/middleware/request_tracker.rb

@@ -120,7 +120,10 @@ class Middleware::RequestTracker
 
   def self.get_data(env, result, timing, request = nil)
     status, headers = result
+
+    # result may be nil if the downstream app raised an exception
     status = status.to_i
+    headers ||= {}
 
     request ||= Rack::Request.new(env)
     helper = Middleware::AnonymousCache::Helper.new(env, request)

spec/lib/middleware/request_tracker_spec.rb

@@ -816,4 +816,22 @@ RSpec.describe Middleware::RequestTracker do
       expect(@data[:background_type]).to eq("message-bus-dontchunk")
     end
   end
+
+  describe "error handling" do
+    before do
+      @original_logger = Rails.logger
+      Rails.logger = @fake_logger = FakeLogger.new
+    end
+
+    after { Rails.logger = @original_logger }
+
+    it "logs requests even if they cause exceptions" do
+      app = lambda { |env| raise RateLimiter::LimitExceeded, 1 }
+      tracker = Middleware::RequestTracker.new(app)
+      expect { tracker.call(env) }.to raise_error(RateLimiter::LimitExceeded)
+      CachedCounting.flush
+      expect(ApplicationRequest.stats).to include("http_total_total" => 1)
+      expect(@fake_logger.warnings).to be_empty
+    end
+  end
 end