Suppress new Style/ReturnNilInPredicateMethodDefinition offenses

Follow up rubocop/rubocop#11967.

This commit suppresses the following new `Style/ReturnNilInPredicateMethodDefinition` offenses:

```console
$ bundle exec rubocop
(snip)

Offenses:

lib/rubocop/cop/performance/fixed_size.rb:81:11: C: [Correctable] Style/ReturnNilInPredicateMethodDefinition:
Use return false instead of return in the predicate method.
          return unless node.array_type?
          ^^^^^^
lib/rubocop/cop/performance/fixed_size.rb:87:11: C: [Correctable] Style/ReturnNilInPredicateMethodDefinition:
Use return false instead of return in the predicate method.
          return unless node.hash_type?
          ^^^^^^

116 files inspected, 2 offenses detected, 2 offenses autocorrectable
```

lib/rubocop/cop/performance/fixed_size.rb

@@ -78,13 +78,13 @@ def allowed_parent?(node)
         end
 
         def contains_splat?(node)
-          return unless node.array_type?
+          return false unless node.array_type?
 
           node.each_child_node(:splat).any?
         end
 
         def contains_double_splat?(node)
-          return unless node.hash_type?
+          return false unless node.hash_type?
 
           node.each_child_node(:kwsplat).any?
         end