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

Fixes rubocop#116 and adds regression tests for rubocop#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.

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

```console
% 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
```

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

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

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#116](https://github.com/rubocop-hq/rubocop-rails/issues/116): Fix an incorrect autocorrect for `Rails/Presence` when `else` branch of ternary operator is not nil. ([@koic][])
+
 ## 2.3.1 (2019-08-26)
 
 ### Bug fixes

lib/rubocop/cop/rails/presence.rb

@@ -119,8 +119,10 @@ def message(node, receiver, other)
         def replacement(receiver, other)
           or_source = if other&.send_type?
                         build_source_for_or_method(other)
-                      else
+                      elsif other.nil? || other.nil_type?
                         ''
+                      else
+                        " || #{other.source}"
                       end
 
           "#{receiver.source}.presence" + or_source

spec/rubocop/cop/rails/presence_spec.rb

@@ -36,6 +36,8 @@
   it_behaves_like 'offense', '!a.present? ? b : a', 'a.presence || b', 1, 1
   it_behaves_like 'offense', 'a.blank? ? b : a', 'a.presence || b', 1, 1
   it_behaves_like 'offense', '!a.blank? ? a : b', 'a.presence || b', 1, 1
+  it_behaves_like 'offense', 'a.present? ? a : 1', 'a.presence || 1', 1, 1
+  it_behaves_like 'offense', 'a.blank? ? 1 : a', 'a.presence || 1', 1, 1
 
   it_behaves_like 'offense',
                   'a(:bar).map(&:baz).present? ? a(:bar).map(&:baz) : nil',