Improve `Verify()` Performance

[JasonBock]

Describe the solution you'd like
I noticed that the verification test for BenchmarkMockNet got a little worse from 7.3.0 to 8.0.0-alpha.1. I think I can improve it. Right now, it generates this:

public override void Verify()
{
  if (this.WasInstanceInvoked)
  {
    var failures = new global::System.Collections.Generic.List<string>();

    failures.AddRange(this.Verify(handlers0));
    failures.AddRange(this.Verify(handlers1));

    if (failures.Count > 0)
    {
      throw new global::Rocks.Exceptions.VerificationException(failures);
    }
  }
}

I think if I change it so I only do Verify() on the handler list if it has members that will make things better, like this:

public override void Verify()
{
  if (this.WasInstanceInvoked)
  {
    var failures = new global::System.Collections.Generic.List<string>();

    if (this.handlers0.Count > 0) { failures.AddRange(this.Verify(this.handlers0)); }
    if (this.handlers1.Count > 0) { failures.AddRange(this.Verify(this.handlers1)); }

    if (failures.Count > 0)
    {
      throw new global::Rocks.Exceptions.VerificationException(failures);
    }
  }
}

Note that I should reference the handler lists with this. because reasons :).

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