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, 5 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index c18403865f..d384aba9c1 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Handle JSON deserialization correctly if the column default from database
+    adapter returns `''` instead of `nil`.
+
+    *Johannes Opper*
+
 *   Ensure that mutations of the array returned from `ActiveRecord::Relation#to_a`
     do not affect the original relation, by returning a duplicate array each time.