Save unnecessary `RuneCountInString`

## Enhancement
`RuneCountInString` can be unexpectedly time-consuming for certain workloads. Since the number of runes must be smaller than or equal to the length of the string, we can fast check the length first. Only calculate the number of runes when necessary.
<img width="588" alt="image" src="https://github.com/pingcap/tidb/assets/31720476/29d5b9fb-79b6-4dc1-82c1-11cda7f460a6">
