Fix improper default value check in tbot's FromCLIConf() (#11169)

The default `join_method` value was improperly assumed to be "" which
caused any onboarding config from the config file to be overwritten.

This uses the correct default and allows bots to be fully onboarded
via config file.

Fixes #11099

tool/tbot/config/config.go

@@ -277,7 +277,7 @@ func FromCLIConf(cf *CLIConf) (*BotConfig, error) {
 	// (CAPath, CAPins, etc follow different codepaths so we don't want a
 	// situation where different fields become set weirdly due to struct
 	// merging)
-	if cf.Token != "" || len(cf.CAPins) > 0 || cf.JoinMethod != "" {
+	if cf.Token != "" || len(cf.CAPins) > 0 || cf.JoinMethod != DefaultJoinMethod {
 		onboarding := config.Onboarding
 		if onboarding != nil && (onboarding.Token != "" || onboarding.CAPath != "" || len(onboarding.CAPins) > 0) || cf.JoinMethod != DefaultJoinMethod {
 			// To be safe, warn about possible confusion.