Describe the bug

Calling LogicalPlan::get_parameter_types() does not return the place holder from SELECT $1;.

To Reproduce

The assertion triggers because no parameters are returned.

use datafusion::error::Result;
use datafusion::prelude::*;

#[tokio::main]
async fn main() -> Result<()> {
    let ctx = SessionContext::new();
    let plan = ctx.state().create_logical_plan("SELECT $1;").await?;
    let params = plan.get_parameter_types()?;
    assert_eq!(params.len(), 1);
    Ok(())
}

Expected behavior

All placeholders are returned.

Additional context

There appears to be a builtin assumption that get_parameter_types only returns placeholders that have a resolved type while filtering out anything with a None data_type:

I can't decide if that's a valid assumption or was just a left over from when things were relying on Placeholders to always have a resolved datatype. Given that the function signature is HashMap<String, Option<DataType>> but values are guaranteed to always be Some, I'm just plain confused.