Handle errors without causes

The method `LoadBalancer#connection_error?` would unwrap errors that
inherit from ActiveRecord::StatementInvalid and
ActionView::Template::Error, in an attempt to see if any connection
errors were wrapped. When given an error without a cause, such as
ActiveRecord::RecordNotUnique, it would fail, as `cause` on the error
returns `nil`. This in turn would manifest in various tests, such as
those for the `ApplicationRecord` model.

This commit fixes these issues by just returning `false` when there is
no error cause.

Changelog: fixed

lib/gitlab/database/load_balancing/load_balancer.rb

@@ -169,7 +169,11 @@ module Gitlab
           when ActiveRecord::StatementInvalid, ActionView::Template::Error
             # After connecting to the DB Rails will wrap query errors using this
             # class.
-            connection_error?(error.cause)
+            if (cause = error.cause)
+              connection_error?(cause)
+            else
+              false
+            end
           when *CONNECTION_ERRORS
             true
           else

spec/lib/gitlab/database/load_balancing/load_balancer_spec.rb

@@ -283,6 +283,12 @@ RSpec.describe Gitlab::Database::LoadBalancing::LoadBalancer, :request_store do
 
       expect(lb.connection_error?(error)).to eq(false)
     end
+
+    it 'returns false for ActiveRecord errors without a cause' do
+      error = ActiveRecord::RecordNotUnique.new
+
+      expect(lb.connection_error?(error)).to eq(false)
+    end
   end
 
   describe '#serialization_failure?' do