ArgumentException description

Document: "ArgumentException The tasks argument contains a null element. -or- The tasks argument is an empty array."
But an empty array does not cause an ArgumentException.
https://referencesource.microsoft.com/#mscorlib/system/threading/Tasks/Task.cs

---
#### Document Details

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* ID: a1940f35-0d7f-ebad-aaae-ef6aaabf8f86
* Version Independent ID: b6ffac9c-44a8-7535-7a82-d87b9046289d
* Content: [Task.WaitAll Method (System.Threading.Tasks)](https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.task.waitall?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1&l=EN-US&k=k(System.Threading.Tasks.Task.WaitAll);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.6);k(DevLang-csharp)&rd=true&view=netframework-4.7.2)
* Content Source: [xml/System.Threading.Tasks/Task.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Threading.Tasks/Task.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**