Use a null object to avoid checks to @instance

In the `ConnectionManagement` class, a pattern we see a lot is the
following one:
```ruby
if @instance
  # do something
else
  # do something else
end
```

This patch aims to simplify this a bit by introducing a null object
(`NullInstance`) which will be returned when `@instance` is null. This
way the rest of the code doesn’t care with which object it’s working and
has just to call the expected methods on it.

.github/workflows/ci.yml

@@ -16,9 +16,12 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ["3.0", "3.1", "3.2"]
-        rails: ["6.0", "6.1", "7.0", "edge"]
+        ruby: ["3.0", "3.1", "3.2", "3.3"]
+        rails: ["6.1", "7.0", "7.1", "edge"]
         ok_to_fail: [""]
+        exclude:
+          - ruby: "3.0"
+            rails: "edge"
     steps:
       - uses: actions/checkout@v2
       - uses: ruby/setup-ruby@v1

.rubocop.yml

@@ -4,3 +4,6 @@ AllCops:
   Exclude:
    - gemfiles/vendor/bundle/**/*
    - vendor/bundle/**/*
+
+Discourse/Plugins:
+  Enabled: false

gemfiles/rails_7.1.gemfile

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+source 'https://rubygems.org'
+
+group :test do
+  gem 'activerecord', '~> 7.1.0'
+  gem 'railties', '~> 7.1.0'
+  gem 'rspec'
+  gem 'sqlite3', '~> 1.4'
+  gem 'byebug'
+  gem 'rubocop'
+  gem 'rubocop-discourse'
+end