[I] Empty strings in CSV files aren't being interpreted as null when using a `Dictionary(_, Utf8)` [datafusion]

[via GitHub]

rumpuslabs opened a new issue, #12041:
URL: https://github.com/apache/datafusion/issues/12041

   ### Describe the bug
   
   Related to #7797
   
   Empty strings in CSV files aren't being interpreted as null when using a 
`Dictionary(_, Utf8)`
   
   ### To Reproduce
   
   Create a simple `input.csv` file like this:
   
   ```
   id,name
   1,
   2,bob
   ```
   
   Run the following code:
   
   ```rust
   #[tokio::main]
   async fn main() -> Result<(), DataFusionError> {
       let ctx = SessionContext::new();
   
       let format = CsvFormat::default();
       let listing_options = ListingOptions::new(Arc::new(format));
       ctx.register_listing_table(
           "input",
           "input.csv",
           listing_options.clone(),
           Some(Arc::new(Schema::new(vec![
               Field::new("id", DataType::Utf8, false),
               Field::new(
                   "name",
                   DataType::Dictionary(Box::new(DataType::UInt8), 
Box::new(DataType::Utf8)),
                   true,
               ),
           ]))),
           None,
       )
       .await?;
   
       let results = ctx
           .table("input")
           .await?
           .filter(col("name").is_not_null())?
           .collect()
           .await?;
   
       let pretty_results = 
arrow::util::pretty::pretty_format_batches(&results)?.to_string();
   
       println!("{}", pretty_results);
   
       Ok(())
   }
   ```
   
   ### Expected behavior
   
   I was expecting the output to look like this:
   
   ```
   +----+------+
   | id | name |
   +----+------+
   | 2  | bob  |
   +----+------+
   ```
   
   But the full dataset is returned instead:
   
   ```
   +----+------+
   | id | name |
   +----+------+
   | 1  |      |
   | 2  | bob  |
   +----+------+
   ```
   
   ### Additional context
   
   Tested on v41.0.0
   
   Replace `DataType::Dictionary(Box::new(DataType::UInt8), 
Box::new(DataType::Utf8))` with `DataType::Utf8` and it works.


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