Fix: HttpMethodMatcherPolicy.ApplyAsync ArgumentNullException (#39759)

* Fix NullArgumentException while creating rejection endpoint

* Update HttpMethodMatcherPolicy.cs

* Apply suggestions from code review

* Update src/Http/Routing/test/UnitTests/Matching/HttpMethodMatcherPolicyTest.cs

Co-authored-by: Pranav K <[email protected]>

Author: z77ma

src/Http/Routing/src/Matching/HttpMethodMatcherPolicy.cs

@@ -162,7 +162,7 @@ public Task ApplyAsync(HttpContext httpContext, CandidateSet candidates)
         if (needs405Endpoint == true)
         {
             // We saw some endpoints coming in, and we eliminated them all.
-            httpContext.SetEndpoint(CreateRejectionEndpoint(methods!.OrderBy(m => m, StringComparer.OrdinalIgnoreCase)));
+            httpContext.SetEndpoint(CreateRejectionEndpoint(methods?.OrderBy(m => m, StringComparer.OrdinalIgnoreCase)));
             httpContext.Request.RouteValues = null!;
         }
 
@@ -398,9 +398,9 @@ public PolicyJumpTable BuildJumpTable(int exitDestination, IReadOnlyList<PolicyJ
         }
     }
 
-    private static Endpoint CreateRejectionEndpoint(IEnumerable<string> httpMethods)
+    private static Endpoint CreateRejectionEndpoint(IEnumerable<string>? httpMethods)
     {
-        var allow = string.Join(", ", httpMethods);
+        var allow = httpMethods is null ? string.Empty : string.Join(", ", httpMethods);
         return new Endpoint(
             (context) =>
             {

src/Http/Routing/test/UnitTests/Matching/HttpMethodMatcherPolicyTest.cs

@@ -151,6 +151,28 @@ public void IEndpointSelectorPolicy_AppliesToNode_EndpointIsNotDynamic_ReturnsFa
         Assert.False(result);
     }
 
+    [Theory]
+    [InlineData(1)]
+    [InlineData(2)]
+    public async Task IEndpointSelectorPolicy_ApplyAsync_ProcessesInvalidCandidate(int candidateNum)
+    {
+        var policy = (IEndpointSelectorPolicy)CreatePolicy();
+
+        var endpoints = new RouteEndpoint[candidateNum];
+        for (int i = 0; i < candidateNum; i++)
+        {
+            endpoints[i] = CreateEndpoint("/", new HttpMethodMetadata(new[] { "DEL" }));
+        }
+
+        var candidates = new CandidateSet(endpoints, new RouteValueDictionary[endpoints.Length], Enumerable.Repeat<int>(-1, candidateNum).ToArray());
+        var httpContext = new DefaultHttpContext();
+
+        await policy.ApplyAsync(httpContext, candidates);
+
+        Assert.Equal(httpContext.GetEndpoint().Metadata, EndpointMetadataCollection.Empty);
+        Assert.True(string.Equals(httpContext.GetEndpoint().DisplayName, Http405EndpointDisplayName, StringComparison.OrdinalIgnoreCase));
+    }
+
     [Fact]
     public void GetEdges_GroupsByHttpMethod()
     {