Return an empty dict if nan values is not provided by the catalog (#1575)

Fixes: #1574

Author: summermousa-vendia

pyiceberg/table/inspect.py

@@ -207,7 +207,7 @@ def _readable_metrics_struct(bound_type: PrimitiveType) -> pa.StructType:
                             "column_sizes": dict(entry.data_file.column_sizes),
                             "value_counts": dict(entry.data_file.value_counts),
                             "null_value_counts": dict(entry.data_file.null_value_counts),
-                            "nan_value_counts": entry.data_file.nan_value_counts,
+                            "nan_value_counts": dict(entry.data_file.nan_value_counts),
                             "lower_bounds": entry.data_file.lower_bounds,
                             "upper_bounds": entry.data_file.upper_bounds,
                             "key_metadata": entry.data_file.key_metadata,