Make GetHeaderValue support HttpContext unavailable (#16654)

* Make GetHeaderValue tolerant for when the http context is not available. Now it just returns null.

* Add unit tests

src/Umbraco.Cms.Api.Delivery/Services/RequestHeaderHandler.cs

@@ -8,11 +8,5 @@ internal abstract class RequestHeaderHandler
 
     protected RequestHeaderHandler(IHttpContextAccessor httpContextAccessor) => _httpContextAccessor = httpContextAccessor;
 
-    protected string? GetHeaderValue(string headerName)
-    {
-        HttpContext httpContext = _httpContextAccessor.HttpContext ??
-                                  throw new InvalidOperationException("Could not obtain an HTTP context");
-
-        return httpContext.Request.Headers[headerName];
-    }
+    protected string? GetHeaderValue(string headerName) => _httpContextAccessor.HttpContext?.Request.Headers[headerName];
 }

tests/Umbraco.Tests.UnitTests/Umbraco.Cms.Api.Delivery/Services/RequestHeaderHandlerTests.cs

@@ -0,0 +1,48 @@
+using Microsoft.AspNetCore.Http;
+using Moq;
+using NUnit.Framework;
+using Umbraco.Cms.Api.Delivery.Services;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Cms.Api.Delivery.Services;
+
+[TestFixture]
+public class RequestHeaderHandlerTests
+{
+    private const string HeaderName = "TestHeader";
+    [Test]
+    public void GetHeaderValue_return_null_when_http_context_is_unavailable()
+    {
+        IHttpContextAccessor httpContextAccessor = Mock.Of<IHttpContextAccessor>();
+
+        var sut = new TestRequestHeaderHandler(httpContextAccessor);
+
+        Assert.IsNull(sut.TestGetHeaderValue(HeaderName));
+    }
+
+    [Test]
+    public void GetHeaderValue_return_header_value_when_http_context_is_available()
+    {
+
+        const string headerValue = "TestValue";
+
+        HttpContext httpContext = new DefaultHttpContext();
+        httpContext.Request.Headers[HeaderName] = headerValue;
+
+        IHttpContextAccessor httpContextAccessor = Mock.Of<IHttpContextAccessor>();
+        Mock.Get(httpContextAccessor).Setup(x => x.HttpContext).Returns(httpContext);
+
+        var sut = new TestRequestHeaderHandler(httpContextAccessor);
+
+        Assert.AreEqual(headerValue, sut.TestGetHeaderValue(HeaderName));
+    }
+}
+
+
+internal class TestRequestHeaderHandler : RequestHeaderHandler
+{
+    public TestRequestHeaderHandler(IHttpContextAccessor httpContextAccessor) : base(httpContextAccessor)
+    {
+    }
+
+    public string? TestGetHeaderValue(string headerName) => base.GetHeaderValue(headerName);
+}