add `apply #[serde_as(deserialize_as = "DefaultOnNull")]`

I removed 
```
#[serde_as(deserialize_as = "DefaultOnNull")]
```
 from each String type in the structure and appended
```
#[apply(
    String => #[serde_as(deserialize_as = "DefaultOnNull")]
)]
```
but it doesn't seem to be working correctly for String types.