Fix an error for Rails/UniqueValidationWithoutIndex

[koic]

Resolves #1022 (comment).

This PR fixes an error for Rails/UniqueValidationWithoutIndex when the presence: true option is used alone for the validates method.

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[rnestler]

Should this really register an offense in this case? Doesn't make sense in my opinion, since there is no uniqueness requirement on the field?

[mobilutz]

I also think this should not raise an error.

Think about this example:

class Post
  belongs_to :user

  validates :user, presence: true
end

class User
  has_many :posts
end

The column user_id in posts can NEVER be unique because a user can have many posts.

[koic]

Thank you for the comments. This is a bug and I'm working now.

[koic]

I've updated this PR.

[ydah]

Ah, #1021 correction was not good enough. Maybe, it should have been as follows.

- return if uniqueness_part(node)&.falsey_literal?
+ return unless uniqueness_part =  uniqueness_part(node)
+ return if uniqueness_part.falsey_literal?

[markijbema]

Tested this PR against our codebase, and it fixed the problem for us. Thank you!