.Net: Minor cleanup of collection nullability. (#2107)

Collections are being checked for null, even when they cannot or should
not ever be null.

Fixes #2064
Fixes #2065

### Description
Removing nullable operators/checks from Skills collections

### Contribution Checklist
- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#dev-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

dotnet/src/Extensions/Planning.SequentialPlanner/SKContextSequentialPlannerExtensions.cs

@@ -59,23 +59,12 @@ public static class SKContextSequentialPlannerExtensions
         string? semanticQuery = null,
         CancellationToken cancellationToken = default)
     {
-        var excludedSkills = config.ExcludedSkills ?? new();
-        var excludedFunctions = config.ExcludedFunctions ?? new();
-        var includedFunctions = config.IncludedFunctions ?? new();
-
-        if (context.Skills == null)
-        {
-            throw new KernelException(
-                KernelException.ErrorCodes.SkillCollectionNotSet,
-                "Skill collection not found in the context");
-        }
-
         var functionsView = context.Skills.GetFunctionsView();
 
         var availableFunctions = functionsView.SemanticFunctions
             .Concat(functionsView.NativeFunctions)
             .SelectMany(x => x.Value)
-            .Where(s => !excludedSkills.Contains(s.SkillName) && !excludedFunctions.Contains(s.Name))
+            .Where(s => !config.ExcludedSkills.Contains(s.SkillName) && !config.ExcludedFunctions.Contains(s.Name))
             .ToList();
 
         List<FunctionView>? result = null;
@@ -104,7 +93,7 @@ public static class SKContextSequentialPlannerExtensions
             result.AddRange(await GetRelevantFunctionsAsync(context, availableFunctions, memories, cancellationToken).ConfigureAwait(false));
 
             // Add any missing functions that were included but not found in the search results.
-            var missingFunctions = includedFunctions
+            var missingFunctions = config.IncludedFunctions
                 .Except(result.Select(x => x.Name))
                 .Join(availableFunctions, f => f, af => af.Name, (_, af) => af);
 

dotnet/src/SemanticKernel.Abstractions/Orchestration/SKContext.cs

@@ -108,7 +108,7 @@ public ISemanticTextMemory Memory
     /// <summary>
     /// Read only skills collection
     /// </summary>
-    public IReadOnlySkillCollection? Skills { get; internal set; }
+    public IReadOnlySkillCollection Skills { get; }
 
     /// <summary>
     /// Access registered functions by skill + name. Not case sensitive.
@@ -119,13 +119,6 @@ public ISemanticTextMemory Memory
     /// <returns>Delegate to execute the function</returns>
     public ISKFunction Func(string skillName, string functionName)
     {
-        if (this.Skills is null)
-        {
-            throw new KernelException(
-                KernelException.ErrorCodes.SkillCollectionNotSet,
-                "Skill collection not found in the context");
-        }
-
         return this.Skills.GetFunction(skillName, functionName);
     }
 

dotnet/src/SemanticKernel/SkillDefinition/SKFunction.cs

@@ -227,9 +227,6 @@ public FunctionView Describe()
     /// <inheritdoc/>
     public async Task<SKContext> InvokeAsync(SKContext context, CompleteRequestSettings? settings = null, CancellationToken cancellationToken = default)
     {
-        // If the function is invoked manually, the user might have left out the skill collection
-        context.Skills ??= this._skillCollection;
-
         if (this.IsSemantic)
         {
             this.AddDefaultValues(context.Variables);