fix: specify roottype in fieldreference

Signed-off-by: MBWhite <whitemat@uk.ibm.com>

Author: mbwhite

datafusion/substrait/src/logical_plan/producer.rs

@@ -18,6 +18,7 @@
 use datafusion::config::ConfigOptions;
 use datafusion::optimizer::analyzer::expand_wildcard_rule::ExpandWildcardRule;
 use datafusion::optimizer::AnalyzerRule;
+use substrait::proto::expression::field_reference::{RootReference, RootType};
 use std::sync::Arc;
 use substrait::proto::expression_reference::ExprType;
 
@@ -2150,7 +2151,7 @@ fn try_to_substrait_field_reference(
                         }),
                     )),
                 })),
-                root_type: None,
+                root_type: Some(RootType::RootReference(RootReference{}) ),
             })
         }
         _ => substrait_err!("Expect a `Column` expr, but found {expr:?}"),
@@ -2192,7 +2193,7 @@ fn substrait_field_ref(index: usize) -> Result<Expression> {
                     }),
                 )),
             })),
-            root_type: None,
+            root_type: Some(RootType::RootReference(RootReference{}) ),
         }))),
     })
 }