[MM-60278] app/config.go: add a nil check for a required field (#28064)

* app/config.go: add a nil check for a required field

* Apply suggestions from code review

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

---------

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/config.go

@@ -249,7 +249,7 @@ func (a *App) HandleMessageExportConfig(cfg *model.Config, appCfg *model.Config)
 	// appropriate value. The rewriting occurs here to ensure it doesn't affect values written to the config file
 	// directly and not through the System Console UI.
 	if *cfg.MessageExportSettings.EnableExport != *appCfg.MessageExportSettings.EnableExport {
-		if *cfg.MessageExportSettings.EnableExport && *cfg.MessageExportSettings.ExportFromTimestamp == int64(0) {
+		if *cfg.MessageExportSettings.EnableExport && model.SafeDereference(cfg.MessageExportSettings.ExportFromTimestamp) == int64(0) {
 			// When the feature is toggled on, use the current timestamp as the start time for future exports.
 			cfg.MessageExportSettings.ExportFromTimestamp = model.NewPointer(model.GetMillis())
 		} else if !*cfg.MessageExportSettings.EnableExport {