fix: Try return type null

Author: Weijun-H

datafusion/functions/src/datetime/now.rs

@@ -15,16 +15,13 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use arrow::datatypes::DataType::Timestamp;
-use arrow::datatypes::TimeUnit::Nanosecond;
-use arrow::datatypes::{DataType, Field, FieldRef};
+use arrow::datatypes::DataType;
 use std::any::Any;
 
 use datafusion_common::{internal_err, Result, ScalarValue};
 use datafusion_expr::simplify::{ExprSimplifyResult, SimplifyInfo};
 use datafusion_expr::{
-    ColumnarValue, Documentation, Expr, ReturnFieldArgs, ScalarUDFImpl, Signature,
-    Volatility,
+    ColumnarValue, Documentation, Expr, ScalarUDFImpl, Signature, Volatility,
 };
 use datafusion_macros::user_doc;
 
@@ -77,17 +74,8 @@ impl ScalarUDFImpl for NowFunc {
         &self.signature
     }
 
-    fn return_field_from_args(&self, _args: ReturnFieldArgs) -> Result<FieldRef> {
-        Ok(Field::new(
-            self.name(),
-            Timestamp(Nanosecond, Some("+00".into())),
-            false,
-        )
-        .into())
-    }
-
     fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
-        internal_err!("return_field_from_args should be called instead")
+        Ok(DataType::Null)
     }
 
     fn invoke_with_args(