[Fix #86] Fix an incorrect autocorrect for Performance/RedundantMerge

[koic]

Fixes #86.

This PR fixes the following incorrect autocorrect for Performance/RedundantMerge when using an empty hash argument.

% cat example.rb
def something
  hash.merge!({}) if blah?
  hash
end

% rubocop --only Performance/RedundantMerge -a
Inspecting 1 file
E

Offenses:

example.rb:2:3: C: [Corrected] Performance/RedundantMerge: Use  instead
of hash.merge!({}).
  hash.merge!({}) if blah?
  ^^^^^^^^^^^^^^^
example.rb:5:1: E: Lint/Syntax: unexpected token $end
(Using Ruby 2.6 parser; configure using TargetRubyVersion parameter,
under AllCops)

1 file inspected, 2 offenses detected, 1 offense corrected

% cat example.rb
def something
   if blah?
  hash
end

There is no good case code to replace an empty hash argument, so this PR changes it to not offense.

---

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