config/default: Set Enabled: false for new RuboCop rules

- The following message appeared when I ran `bundle exec rubocop` on this
  project. Since we're setting everything to `Enabled: false` to mimic
  `DisabledByDefault` for now, let's make this message go away. We almost
  certainly also don't want to blanket enable all new cops, so I've not
  added that configuration.

```
The following cops were added to RuboCop, but are not configured. Please set Enabled to either `true` or `false` in your `.rubocop.yml` file.

Please also note that you can opt-in to new cops by default by adding this to your config:
  AllCops:
    NewCops: enable

Lint/EmptyBlock: # new in 1.1
  Enabled: true
Lint/EmptyClass: # new in 1.3
  Enabled: true
Lint/EmptyInPattern: # new in 1.16
  Enabled: true
For more information: https://docs.rubocop.org/rubocop/versioning.html
```

config/default.yml

@@ -429,6 +429,15 @@ Lint/EachWithObjectArgument:
 Lint/ElseLayout:
   Enabled: true
 
+Lint/EmptyBlock:
+  Enabled: false
+
+Lint/EmptyClass:
+  Enabled: false
+
+Lint/EmptyInPattern:
+  Enabled: false
+
 Lint/EmptyConditionalBody:
   Enabled: false