Fix an error for Performance/RedundantMerge

Follow up rubocop/rubocop-performance#68 (comment).

#68 didn't solve an issue #67 with `Performance/RedundantMerge`.
This PR is corrected by adding a reproduction test.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#74](https://github.com/rubocop-hq/rubocop-performance/pull/74): Fix an error for `Performance/RedundantMerge` when `MaxKeyValuePairs` option is set to `null`. ([@koic][])
+
 ## 1.4.1 (2019-07-29)
 
 ### Bug fixes

lib/rubocop/cop/performance/redundant_merge.rb

@@ -132,7 +132,7 @@ def indent_width
         end
 
         def max_key_value_pairs
-          Integer(cop_config['MaxKeyValuePairs']) || 2
+          Integer(cop_config['MaxKeyValuePairs'] || 2)
         end
 
         # A utility class for checking the use of values within an

spec/rubocop/cop/performance/redundant_merge_spec.rb

@@ -261,4 +261,18 @@
       RUBY
     end
   end
+
+  context 'when MaxKeyValuePairs is set to nil' do
+    let(:cop_config) do
+      { 'MaxKeyValuePairs' => nil }
+    end
+
+    it 'does not raise `TypeError`' do
+      expect_offense(<<~RUBY)
+        hash = {}
+        hash.merge!(a: 1, b: 2)
+        ^^^^^^^^^^^^^^^^^^^^^^^ Use `hash[:a] = 1; hash[:b] = 2` instead of `hash.merge!(a: 1, b: 2)`.
+      RUBY
+    end
+  end
 end