[iceberg] Make com.facebook.presto.iceberg.PrestoIcebergSchema.aliases not null

Author: imjalpreet

presto-iceberg/src/main/java/com/facebook/presto/iceberg/PrestoIcebergSchema.java

@@ -45,7 +45,7 @@ public PrestoIcebergSchema(
         this.schemaId = schemaId;
         this.columns = ImmutableList.copyOf(requireNonNull(columns, "columns is null"));
         this.columnNameToIdMapping = ImmutableMap.copyOf(requireNonNull(columnNameToIdMapping, "columnNameToIdMapping is null"));
-        this.aliases = aliases != null ? ImmutableMap.copyOf(aliases) : null;
+        this.aliases = aliases != null ? ImmutableMap.copyOf(aliases) : ImmutableMap.of();
         this.identifierFieldIds = ImmutableSet.copyOf(requireNonNull(identifierFieldIds, "identifierFieldIds is null"));
     }
 

presto-iceberg/src/main/java/com/facebook/presto/iceberg/SchemaConverter.java

@@ -45,7 +45,7 @@ public static Schema toIcebergSchema(PrestoIcebergSchema schema)
                 schema.getColumns().stream()
                         .map(nestedField -> toIcebergNestedField(nestedField, schema.getColumnNameToIdMapping()))
                         .collect(toImmutableList()),
-                schema.getAliases(),
+                schema.getAliases().isEmpty() ? null : schema.getAliases(),
                 schema.getIdentifierFieldIds());
     }
 }