Address `Use assert_nil if expecting nil` warnings

[yahonda]

This pull request suppresses Use assert_nil if expecting nil warnings.

Environment

• Arel master branch
• ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]
• minitest (5.10.1)

Steps to reproduce:

$ bundle exec ruby -w -I"lib:lib:test" test/test_select_manager.rb
Run options: --seed 11993

# Running:

.........................................................Use assert_nil if expecting nil from /home/yahonda/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/minitest-5.10.1/lib/minitest/spec.rb:23:in `must_equal'. This will fail in MT6.
................................................Use assert_nil if expecting nil from /home/yahonda/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/minitest-5.10.1/lib/minitest/spec.rb:23:in `must_equal'. This will fail in MT6.
.......

Finished in 0.048920s, 2289.4336 runs/s, 2820.9093 assertions/s.

112 runs, 138 assertions, 0 failures, 0 errors, 0 skips
$

[rails-bot]

r? @tenderlove

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[yahonda]

Thanks for the useful information. Updated to use must_be_nil and squashed.

[kaspth]

Thanks!