Add unset_nil option to ConfigLoader.merge_with_default

### Summary

Follow up #6942.

`unset_nil` was introduced to RuboCop core at #6942.

This causes the following warning to be displayed for
`TargetRailsVersion` and `Database` where `null` (or `~`) is
specified in the RuboCop Rails `config/default.yml` configuration.

- https://github.com/rubocop-hq/rubocop-rails/blob/5180fa2c25cef467eececd9c80739f7cad6adbbf/config/default.yml#L11
- https://github.com/rubocop-hq/rubocop-rails/blob/5180fa2c25cef467eececd9c80739f7cad6adbbf/config/default.yml#L89

```console
% cd path/to/rubocop-rails
% bundle exec rake

(snip)

Warning: AllCops does not support TargetRailsVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion

Warning: Rails/BulkChangeTable does not support Database parameter.

Supported parameters are:

  - Enabled
  - SupportedDatabases
  - Include
```

This PR aims to prevent this issue by the following work around to RuboCop Rails.
I will open a PR to RuboCop Rails if this PR is accepted.

```diff
diff --git a/lib/rubocop/rails/inject.rb b/lib/rubocop/rails/inject.rb
index abb715ea7..8d1ffa371 100644
--- a/lib/rubocop/rails/inject.rb
+++ b/lib/rubocop/rails/inject.rb
@@ -10,7 +10,7 @@ module RuboCop
         hash = ConfigLoader.send(:load_yaml_configuration, path)
         config = Config.new(hash, path)
         puts "configuration from #{path}" if ConfigLoader.debug?
-        config = ConfigLoader.merge_with_default(config, path)
+        config = ConfigLoader.merge_with_default(config, path, unset_nil: false)
         ConfigLoader.instance_variable_set(:@default_configuration, config)
       end
     end
```

### Other Information

The following example app code reproduces this issue using
RuboCop Rails master code.

```ruby
# Gemfile
source "https://rubygems.org"

git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }

gem 'rubocop', github: 'rubocop-hq/rubocop'
gem 'rubocop-rails', github: 'rubocop-hq/rubocop-rails'
```

```ruby
# app/models/user.rb
class User < ActiveRecord::Base
end
```

```yaml
# .rubocop.yml
AllCops:
  TargetRailsVersion: 5.2
```

```
% bunlde install
% bundle exec rubocop --only Rails/ApplicationRecord --require rubocop-rails
Warning: AllCops does not support TargetRailsVersion parameter.

Supported parameters are:

  - RubyInterpreters
  - Include
  - Exclude
  - DefaultFormatter
  - DisplayCopNames
  - DisplayStyleGuide
  - StyleGuideBaseURL
  - ExtraDetails
  - StyleGuideCopsOnly
  - EnabledByDefault
  - DisabledByDefault
  - UseCache
  - MaxFilesInCache
  - CacheRootDirectory
  - AllowSymlinksInCacheRootDirectory
  - TargetRubyVersion

Inspecting 2 files
.C

Offenses:

app/models/user.rb:1:14: C: Rails/ApplicationRecord: Models should
subclass ApplicationRecord.
class User < ActiveRecord::Base
             ^^^^^^^^^^^^^^^^^^

2 files inspected, 1 offense detected
```

This means that an unexpected warning is displayed during a migration
period until RuboCop Rails cops is removed from RuboCop core.

lib/rubocop/config_loader.rb

@@ -120,8 +120,8 @@ def default_configuration
       # If AllCops::EnabledByDefault is true, it changes the Enabled params
       # so that only cops explicitly disabled in user configuration are
       # disabled.
-      def merge_with_default(config, config_file)
-        resolver.merge_with_default(config, config_file)
+      def merge_with_default(config, config_file, unset_nil: true)
+        resolver.merge_with_default(config, config_file, unset_nil: unset_nil)
       end
 
       def add_inheritance_from_auto_generated_file

lib/rubocop/config_loader_resolver.rb

@@ -55,7 +55,7 @@ def resolve_inheritance_from_gems(hash, gems)
     # only cops from user configuration are enabled. If
     # AllCops::EnabledByDefault is true, it changes the Enabled params so that
     # only cops explicitly disabled in user configuration are disabled.
-    def merge_with_default(config, config_file)
+    def merge_with_default(config, config_file, unset_nil:)
       default_configuration = ConfigLoader.default_configuration
 
       disabled_by_default = config.for_all_cops['DisabledByDefault']
@@ -71,7 +71,8 @@ def merge_with_default(config, config_file)
         config = handle_disabled_by_default(config, default_configuration)
       end
 
-      opts = { inherit_mode: config['inherit_mode'] || {}, unset_nil: true }
+      opts = { inherit_mode: config['inherit_mode'] || {},
+               unset_nil: unset_nil }
       Config.new(merge(default_configuration, config, opts), config_file)
     end