Prevent DivideByZeroException in EventCounter.ToString() (dotnet/coreclr#27581)

Prevent DivideByZeroException in EventCounter.ToString() (dotnet/coreclr#27581)

* Prevent DivideByZeroException in EventCounter.ToString()

Maybe it should print something other than 0. Maybe "Mean" should be excluded. But at least with this it doesn't divide by zero. Ternary operator in interpolated string is not super readable; didn't know if making the code more descriptive would have performance implications.

* Update src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventCounter.cs

Co-Authored-By: Stephen Toub <stoub@microsoft.com>
Commit migrated from https://github.com/dotnet/coreclr/commit/a187c727956c8ed30635f75450e7dfc38f2ccf38