Fix ArgumentNullException when PropertiesDictionary is used and comparer is null

[eddynaka]

Fixes #2481
This change will fix the ArgumentNullException when passing a null comparer to the ConcurrentDictionary when using the TypedPropertiesDictionary.

The issue:

1. sarif-sdk/src/Sarif.WorkItems/SarifWorkItemContext.cs

   Lines 12 to 22 in ea44355

   	public class SarifWorkItemContext : PropertiesDictionary, IDisposable
   	{
   	public SarifWorkItemContext()
   	{
   	this.CurrentProvider = FilingClient.SourceControlProvider.AzureDevOps;
   	this.AssemblyLocationMap = new Dictionary<string, string>();
   	}
   	public SarifWorkItemContext(SarifWorkItemContext initializer) : base(initializer)
   	{
   	}

2. sarif-sdk/src/Sarif/PropertiesDictionary.cs

   Lines 20 to 31 in ea44355

   	public class PropertiesDictionary : TypedPropertiesDictionary<object>
   	{
   	internal const string DEFAULT_POLICY_NAME = "default";
   	public PropertiesDictionary() : this(null)
   	{
   	}
   	public PropertiesDictionary(PropertiesDictionary initializer) :
   	this(initializer, null)
   	{
   	}

3. sarif-sdk/src/Sarif/TypedPropertiesDictionary.cs

   Lines 16 to 31 in 503fe29

   	public class TypedPropertiesDictionary<T> : ConcurrentDictionary<string, T>, IMarker where T : new()
   	{
   	public TypedPropertiesDictionary() : this(null, StringComparer.Ordinal)
   	{
   	}
   	public TypedPropertiesDictionary(PropertiesDictionary initializer, IEqualityComparer<string> comparer) : base(comparer)
   	{
   	if (initializer != null)
   	{
   	foreach (string key in initializer.Keys)
   	{
   	this[key] = (T)initializer[key];
   	}
   	}
   	}

With that, on (3), the second constructor will receive a null comparer, generating the ArgumentNullException.

[eddynaka]

net48

To simulate the issue, we need to change to targetFramework 48. It will happen every time.

[eddynaka]

As a side note, I also tried to add net48 for all the other test projects to enhance our testing capabilities BUT the pipeline is timing out. So, we have to investigate why is that happening.

[eddynaka]

Also included a simple and focused test to prevent us from facing this issue again.

[eddynaka]

StringComparer

As I mentioned in the description, when this is instantiated by the SarifWorkItemContext, the comparer will be null.

So, to use the same strategy as the constructor above, I'm defaulting the comparer to StringComparer.Ordinal.

[michaelcfanning]

Yes! Good fix. Did you confirm that this is the behavior for Dictionary<string, T> by examining the reference implementation? Ideally, we would match the behavior of that concrete type.

[michaelcfanning]

used

'is instantiated with a null comparer.'

[michaelcfanning]