Raise ArgumentError when passing nil to Relation#merge

nil or false should not be valid argument to the merge method.

Closes #12264

activerecord/CHANGELOG.md

+*   Raise `ArgumentError` when passing `nil` or `false` to `Relation#merge`.
+
+    These are not valid values to merge in a relation so it should warn the users
+    early.
+
+    *Rafael Mendonça França*
+
 *   Use `SCHEMA` instead of `DB_STRUCTURE` for specifiying structure file.
 
     This makes the db:structure tasks consistent with test:load_structure.

activerecord/lib/active_record/relation/spawn_methods.rb

@@ -32,7 +32,7 @@ def merge(other)
       elsif other
         spawn.merge!(other)
       else
-        self
+        raise ArgumentError, "invalid argument: #{other.inspect}."
       end
     end
 

activerecord/test/cases/relation_test.rb

@@ -168,6 +168,22 @@ def test_references_values_dont_duplicate
       assert_raises(ArgumentError) { Relation::HashMerger.new(nil, omg: 'lol') }
     end
 
+    test 'merging nil or false raises' do
+      relation = Relation.new(FakeKlass, :b, nil)
+
+      e = assert_raises(ArgumentError) do
+        relation = relation.merge nil
+      end
+
+      assert_equal 'invalid argument: nil.', e.message
+
+      e = assert_raises(ArgumentError) do
+        relation = relation.merge false
+      end
+
+      assert_equal 'invalid argument: false.', e.message
+    end
+
     test '#values returns a dup of the values' do
       relation = Relation.new(FakeKlass, :b, nil).where! :foo
       values   = relation.values