Fix Performance/Count cop error on empty selector block

Something like

```ruby
collection.reject { }.size
```

would lead to an error

Author: viralpraxis

changelog/fix_performance_count_cop_error_on_empty_selector_block.md

@@ -0,0 +1 @@
+* [#497](https://github.com/rubocop/rubocop-performance/pull/497): Fix `Performance/Count` cop error on empty selector block. ([@viralpraxis][])

lib/rubocop/cop/performance/count.rb

@@ -117,16 +117,21 @@ def negate_block_pass_reject(corrector, node)
 
         def negate_block_reject(corrector, node)
           target =
-            if node.receiver.body.begin_type?
+            if node.receiver.body&.begin_type?
               node.receiver.body.children.last
             else
               node.receiver.body
             end
-          corrector.replace(target, negate_expression(target))
+
+          corrector.replace(target || node.receiver.loc.end.adjust(end_pos: -1), negate_expression(target))
         end
 
         def negate_expression(node)
-          "!(#{node.source})"
+          if node
+            "!(#{node.source})"
+          else
+            'true'
+          end
         end
 
         def negate_block_pass_as_inline_block(node)

spec/rubocop/cop/performance/count_spec.rb

@@ -353,4 +353,17 @@ def count(&block)
       RUBY
     end
   end
+
+  context 'with `reject` with empty block body' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        array.reject {}.size
+              ^^^^^^^^^^^^^^ Use `count` instead of `reject...size`.
+      RUBY
+
+      expect_correction(<<~RUBY)
+        array.count {true}
+      RUBY
+    end
+  end
 end