Make sure the attributes with value nil are not converted to { } when they are being merged at the same precedence level.

Problem: 

If one has below in their roles / cookbooks: 

1-) In the role:

```json
default_attributes:  
  something: 
    foo: 
      some_path: null
```

2-) In the cookbook attributes:

```ruby
default['something']['foo'] =  { 'some_other_path' => "crayons" }
```

And when you iterate on node.default['something']['foo'] as below

```ruby
node.default['something']['foo'].each do |path, value|
  puts "Path is #{path}. Value is #{value}"
end
```

With 10.30.0.rc.1 you will get: 

```ruby
Path is some_other_path. Value is crayons
Path is some_path. Value is {}
```

Expected value you should get is: 

```ruby
Path is some_other_path. Value is crayons
Path is some_path. Value is 
```