Fix bug in JSON deserialization when column default is an empty string

When `ActiveRecord::Coders::JSON` serialization is used and the default of the column returns `''` it raises the following error:

```
JSON::ParserError: A JSON text must at least contain two octets!
```

If MySQL is running in non-strict mode, it returns an empty string as column default for a text column:

```ruby
def extract_default
  if blob_or_text_column?
    @default = null || strict ? nil : ''
  end
end
```

Since `''` is invalid JSON, there shouldn't be an attempt to parse it, it should be treated like nil.
ActiveRecord::Coders::JSON should behave consistently for all possible non-user-set column default values.

1 files changed, 1 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/coders/json.rb b/activerecord/lib/active_record/coders/json.rb
index 75d3bfe625..cb185a881e 100644
--- a/activerecord/lib/active_record/coders/json.rb
+++ b/activerecord/lib/active_record/coders/json.rb
@@ -6,7 +6,7 @@ module ActiveRecord
       end
 
       def self.load(json)
-        ActiveSupport::JSON.decode(json) unless json.nil?
+        ActiveSupport::JSON.decode(json) unless json.blank?
       end
     end
   end