Avoid setting incompatible default value for StringValue sources (dotnet#52895)

Author: captainsafia

src/Http/Http.Extensions/gen/StaticRouteHandlerModel/Emitters/EndpointParameterEmitter.cs

@@ -33,7 +33,7 @@ internal static void EmitQueryOrHeaderParameterPreparation(this EndpointParamete
         }
         else if (endpointParameter.IsOptional)
         {
-            codeWriter.WriteLine($"var {endpointParameter.EmitTempArgument()} = {endpointParameter.EmitAssigningCodeResult()}.Count > 0 ? (string?){endpointParameter.EmitAssigningCodeResult()} : {endpointParameter.DefaultValue};");
+            codeWriter.WriteLine($"var {endpointParameter.EmitTempArgument()} = {endpointParameter.EmitAssigningCodeResult()}.Count > 0 ? (string?){endpointParameter.EmitAssigningCodeResult()} : null;");
         }
         else if (endpointParameter.IsStringValues)
         {

src/Http/Http.Extensions/test/RequestDelegateGenerator/RequestDelegateCreationTests.QueryParameters.cs

@@ -125,6 +125,20 @@ public async Task MapAction_MultipleStringParam_StringReturn()
         await VerifyAgainstBaselineUsingFile(compilation);
     }
 
+    [Fact]
+    public async Task MapAction_ExplicitParsableParameter_StringReturn()
+    {
+        var (results, compilation) = await RunGeneratorAsync("""
+app.MapGet("/hello", ([FromQuery] int p1 = 10) => $"{p1}");
+""");
+        var endpoint = GetEndpointFromCompilation(compilation);
+
+        var httpContext = CreateHttpContext();
+
+        await endpoint.RequestDelegate(httpContext);
+        await VerifyResponseBodyAsync(httpContext, "10");
+    }
+
     public static object[][] MapAction_ExplicitQueryParam_NameTest_Data
     {
         get