Remove text default treated as an empty string in non-strict mode

Strict mode controls how MySQL handles invalid or missing values in
data-change statements such as INSERT or UPDATE. If strict mode is not
in effect, MySQL inserts adjusted values for invalid or missing values
and produces warnings.

```ruby
  def test_mysql_not_null_defaults_non_strict
    using_strict(false) do
      with_mysql_not_null_table do |klass|
        record = klass.new
        assert_nil record.non_null_integer
        assert_nil record.non_null_string
        assert_nil record.non_null_text
        assert_nil record.non_null_blob

        record.save!
        record.reload

        assert_equal 0,  record.non_null_integer
        assert_equal "", record.non_null_string
        assert_equal "", record.non_null_text
        assert_equal "", record.non_null_blob
      end
    end
  end
```

It is inconsistent with other types that only text/blob defaults treated
as an empty string. This commit fixes the inconsistency.

1 files changed, 3 insertions, 4 deletions

diff --git a/activerecord/test/cases/column_definition_test.rb b/activerecord/test/cases/column_definition_test.rb
index 989beaa5c8..a65bb89052 100644
--- a/activerecord/test/cases/column_definition_test.rb
+++ b/activerecord/test/cases/column_definition_test.rb
@@ -60,14 +60,13 @@ module ActiveRecord
         end
 
         def test_should_not_set_default_for_blob_and_text_data_types
-          text_type = MySQL::TypeMetadata.new(
-            SqlTypeMetadata.new(type: :text))
+          text_type = MySQL::TypeMetadata.new(SqlTypeMetadata.new(type: :text))
 
           text_column = MySQL::Column.new("title", nil, text_type)
-          assert_equal nil, text_column.default
+          assert_nil text_column.default
 
           not_null_text_column = MySQL::Column.new("title", nil, text_type, false)
-          assert_equal "", not_null_text_column.default
+          assert_nil not_null_text_column.default
         end
 
         def test_has_default_should_return_false_for_blob_and_text_data_types