Items are not always added to a SkipList

[Hoodedgull]

Describe the bug
When attempting to add an item to the SkipList, it is not always added. This seems to be caused by the fact that the _getNextLevel() method may return 0, in which case the for loop on line 138 that adds elements to the SkipList is not entered because lvl is 0.

To Reproduce
It is a bit difficult to reproduce, since a SkipList is a probabilistic data structure, so the behaviour will not be the same every time it is used. However, the following should work:

1. Create a unit test
2. Create a SkipList in that test
3. Add a random number to the list and call list.Contains(number)
4. Assert that this is true because you just added the number
5. Run that test 1000 times

Expected behavior
I expect that after adding a number to a list of integers, the list will contain that number

Environmnet:

• OS: Windows 10
• .NET Version: .NET Core v3.1.101
• IDE: Visual Studio Enterprise 2019

Additional context
If the Random object in the SkipList is initialized with seed 123, the first call to Add will not insert the element

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