Ignore empty condition on #construct_relation_for_exists

At fc0e3354af,

```rb
relation = relation.where(conditions)
```

was rewritten to:

```rb
relation.where!(condition)
```

This change accidentally changed the result of `Topic.exists?({})` from true to false.

To fix this regression, first I moved the blank check logic (`opts.blank?`) from `#where` to `#where!`,
because I thought `#where!` should be identical to `#where`, except that instead of returning a new relation,
it adds the condition to the existing relation.

But on second thought after some discussion on https://github.com/rails/rails/pull/34329,
I started to think that just fixing `#construct_relation_for_exists` is more preferable
than changing `#where` and `#where!`.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -363,7 +363,7 @@ module ActiveRecord
 
         case conditions
         when Array, Hash
-          relation.where!(conditions)
+          relation.where!(conditions) unless conditions.empty?
         else
           relation.where!(primary_key => conditions) unless conditions == :none
         end

activerecord/test/cases/finder_test.rb

@@ -246,6 +246,10 @@ class FinderTest < ActiveRecord::TestCase
     assert_equal true, Topic.first.replies.exists?
   end
 
+  def test_exists_with_empty_hash_arg
+    assert_equal true, Topic.exists?({})
+  end
+
   # Ensure +exists?+ runs without an error by excluding distinct value.
   # See https://github.com/rails/rails/pull/26981.
   def test_exists_with_order_and_distinct