Fix null reference exception in GenFragmentationPercent method

[Copilot]

This PR fixes a null reference exception that can occur in the GenFragmentationPercent(Gens gen) method when HeapStats is null.

Issue

The GenFragmentationPercent method calls GenSizeAfterMB, which accesses properties on HeapStats without checking if it's null, causing a null reference exception when HeapStats is null.

Fix

Added a null check at the beginning of the GenSizeAfterMB method that returns double.NaN when HeapStats is null. This ensures that:

1. No null reference exception is thrown
2. NaN propagates through the calculations in GenFragmentationPercent
3. The behavior matches the requirement specified in the issue comments

The change is minimal and preserves the existing code structure.

public double GenSizeAfterMB(Gens gen)
{
    if (HeapStats == null)
    {
        return double.NaN;
    }
    
    // Rest of the method remains unchanged
}

Fixes #2012.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• crl3.digicert.com
  • Triggering command: dotnet build (dns block)
• ocsp.digicert.com
  • Triggering command: dotnet build (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

---

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[mrsharm]

LGTM! Will highlight if this fails again.

[brianrob]

Copilot Post-Mortem:

• This one looks right on the first time around. No tweaks needed.

[mconnew]