[Fix #116] Fix an incorrect autocorrect for Rails/Prensence

[koic]

Fixes #116 and adds regression tests for #115.

This PR fixes an incorrect autocorrect for Rails/Presence when else branch of ternary operator is not nil.

The following is a reproduction procedure.

# example.rb
a.blank? ? 1 : a

% bundle exec rubocop --only Rails/Presence -a
Inspecting 1 file
C

Offenses:

example.rb:1:1: C: [Corrected] Rails/Presence: Use a.presence instead of
a.blank? ? 1 : a.
a.blank? ? 1 : a
^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

-a.blank? ? 1 : a
+a.presence

This PR will auto-correct it to a.presence || 1.

---

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