Test duration shouldn't be shown as 0 if no value was provided

[Youssef1313]

Example:

sdk/src/Cli/dotnet/Commands/Test/MTP/TestApplicationHandler.cs

Line 146 in 7538a89

TimeSpan.FromTicks(testResult.Duration ?? 0),

If duration was null, we show it as "0". That doesn't look correct. We shouldn't print the duration at all in this case, because it's simply unknown.