really check if collation config is empty

Author: jeewonhh

src/function/aggregate/distributive/minmax.cpp

@@ -336,7 +336,8 @@ unique_ptr<FunctionData> BindMinMax(ClientContext &context, AggregateFunction &f
 		auto str_collation = StringType::GetCollation(arguments[0]->return_type);
 		Value collation_val;
 		context.TryGetCurrentSetting("default_collation", collation_val);
-		if (!str_collation.empty() || !collation_val.IsNull()) {
+		string collation = collation_val.ToString();
+		if (!str_collation.empty() || !collation.empty()) {
 			// If aggr function is min/max and uses collations, replace bound_function with arg_min/arg_max
 			// to make sure the result's correctness.
 			string function_name = function.name == "min" ? "arg_min" : "arg_max";

src/planner/collation_binding.cpp

@@ -20,7 +20,7 @@ bool PushVarcharCollation(ClientContext &context, unique_ptr<Expression> &source
 	if (collation.empty()) {
 		Value collation_val;
 		context.TryGetCurrentSetting("default_collation", collation_val);
-		collation = collation_val.IsNull() ? string() : collation_val.ToString();
+		collation = collation_val.ToString();
 	}
 	collation = StringUtil::Lower(collation);
 	// bind the collation