Guard createRefCodeLabelLink against null frontendCode

[TaprootFreak]

Summary

• Fixes TypeError: Cannot read properties of null (reading 'toLowerCase') in SocialMediaService that spiked on dfxdev (14:35) and dfxprd (14:39) on 2026-04-17.
• createRefCodeLabelLink() in socialmedia/socialmedia.helper.ts assumed frontendCode is always a non-null string, but incoming savings updates can carry a null/undefined value.
• Adds an early return with empty string — consistent with the existing !refCode guard further down.

Test plan

• Watch Grafana Log Viewer (compose_service=jdm-api, level=error) after merge+deploy — expect the SocialMediaService toLowerCase errors to disappear on both dfxdev and dfxprd.
• Confirm Telegram/Twitter saving-update messages still render ref-code links for non-null frontendCodes (Juice Wallet, Cake Wallet, generic ref codes).