do not create wait groups for slices with len less than 2

[Alexfilus]

Little improvements for parallel package.

[shivamrazorpay]

@Alexfilus why is the improvement helpful ?? Can you give example or any article from where you get the idea to add this change ?

[Alexfilus]

In my project I am using lop.Map for processing data from db. And I don't know how many row I will get. From 0 to 20.
And in case of zero row lop.Map is creating unnecessary waitgroup. In case of 1 row lop.Map is creating unnecessary waitgroup and goroutine. It is just useless work and memory allocations.