[12.x] Optimise join and set functions in Arr class

[JordanchoEftimov]

Improve Arr::set and Arr::join for clarity and performance

• Optimized Arr::set to handle single-key cases early, reducing unnecessary loop execution and improving readability.
• Optimized Arr::join by storing the count of the array once instead of calling count() multiple times, improving performance slightly and making the logic cleaner.

[JordanchoEftimov]

Closed and reopened a new one here #56043