Handle NULL options column in vreplication readers

The options column was changed to DEFAULT NULL, so existing rows
or rows without an explicit options value can have NULL. This
caused JSON parse failures ("unexpected end of JSON input") since
NULL becomes an empty string. Fall back to "{}" when empty.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>
Signed-off-by: Tanjin Xu <tanjin.xu@slack-corp.com>

Author: tanjinx

go/vt/binlog/binlogplayer/binlog_player.go

@@ -612,6 +612,9 @@ func ReadVRSettings(dbClient DBClient, uid int32) (VRSettings, error) {
 		return VRSettings{}, fmt.Errorf("failed to parse defer_secondary_keys column: %v", err)
 	}
 	options := vrRow.AsString("options", "{}")
+	if options == "" {
+		options = "{}"
+	}
 	var workflowOptions vtctldata.WorkflowOptions
 	if err := json.Unmarshal([]byte(options), &workflowOptions); err != nil {
 		return VRSettings{}, fmt.Errorf("failed to parse options column: %v", err)

go/vt/vttablet/tabletmanager/vreplication/controller.go

@@ -79,7 +79,7 @@ type controller struct {
 
 func processWorkflowOptions(params map[string]string) (*vttablet.VReplicationConfig, error) {
 	options, ok := params["options"]
-	if !ok {
+	if !ok || options == "" {
 		options = "{}"
 	}
 	var workflowOptions vtctldata.WorkflowOptions