Using NullString, NullJSON structs while writing to spanner

## Expected Behavior
Change the structs that we use to write to spanner to use the NullString, [NullJSON](https://github.com/googleapis/google-cloud-go/blob/4537b45d2611ce480abfb5d186b59e7258ec872c/spanner/value.go#L774) etc fields instead of the raw data directly. These fields have been added to handle scenarios where the data in that columns is actually Null, the client libraries handle the write differently.

## Actual Behavior
When we get data of a given type, we convert it and write to spanner as is. We do not check for null values of these columns.