[FIX] Adjusted livechat fallbacks to take null values into account (#26909)

* [FIX] Adjusted livechat fallbacks to take null values into account

* [FIX] Adjusted to clear defaultAgent when department is changed

* [FIX] Fixed default guest info not being displayed

* [FIX] Adjusted widget default values logic

* [FIX] Removing unnecessary check

* [FIX] Changed default values logic again to account for changes via api

Co-authored-by: Tasso Evangelista <tasso.evangelista@rocket.chat>