.Net: Update PlannerConfigBase.cs (#2954)

### Motivation and Context

To keep code clean

### Description

Remove unnecessary init

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [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#development-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/SemanticKernel.Core/Planning/PlannerConfigBase.cs

@@ -17,7 +17,7 @@ public abstract class PlannerConfigBase
     /// <summary>
     /// Delegate to get the prompt template string.
     /// </summary>
-    public Func<string>? GetPromptTemplate { get; set; } = null;
+    public Func<string>? GetPromptTemplate { get; set; }
 
     /// <summary>
     /// A list of plugins to exclude from the plan creation request.