Fix config enum entries for remote data fetcher

config_enum entries, which are used for GUC values, need to contain
the end mark. Missing the end mark in the data fetcher entries
resulted in crashing TimescaleDB in Release on Windows. This commit
fixes the end mark for data fetcher config_enum entries.

Fixes #2775

src/guc.c

@@ -35,7 +35,9 @@ static const struct config_enum_entry telemetry_level_options[] = {
 };
 
 static const struct config_enum_entry remote_data_fetchers[] = {
-	{ "rowbyrow", RowByRowFetcherType, false }, { "cursor", CursorFetcherType, false }
+	{ "rowbyrow", RowByRowFetcherType, false },
+	{ "cursor", CursorFetcherType, false },
+	{ NULL, 0, false }
 };
 
 bool ts_guc_enable_optimizations = true;