[README] Hotfix for section links 500 error (#9233)

* check for null object

* README section links 500 error hotfix

* remove incorrect tests from MarkdownServiceFacts.cs

* remove accidental changes to Web.config

* remove accidental changes to Web.config

* add unit test

* change operator for null check

Author: keylime-unicorn

src/NuGetGallery/Services/MarkdownService.cs

@@ -268,7 +268,7 @@ private RenderedMarkdownResult GetHtmlFromMarkdownMarkdig(string markdownString,
                                 // Allow only http or https links in markdown. Transform link to https for known domains.
                                 if (!PackageHelper.TryPrepareUrlForRendering(linkInline.Url, out string readyUriString))
                                 {
-                                    if (!linkInline.Url.StartsWith("#")) //allow internal section links
+                                    if (linkInline.Url != null && !linkInline.Url.StartsWith("#")) //allow internal section links
                                     {
                                         linkInline.Url = string.Empty;
                                     }

tests/NuGetGallery.Facts/Services/MarkdownServiceFacts.cs

@@ -226,6 +226,7 @@ Some text
             [Theory]
             [InlineData("This is a http://www.google.com URL and https://www.google.com", "<p>This is a <a href=\"http://www.google.com/\" rel=\"noopener noreferrer nofollow\">http://www.google.com</a> URL and <a href=\"https://www.google.com/\" rel=\"noopener noreferrer nofollow\">https://www.google.com</a></p>")]
             [InlineData("# This is a heading\n[Link](#this-is-a-heading)", "<h2 id=\"this-is-a-heading\">This is a heading</h2>\n<p><a href=\"#this-is-a-heading\" rel=\"noopener noreferrer nofollow\">Link</a></p>")]
+            [InlineData("# Heading\n[Heading]", "<h2 id=\"heading\">Heading</h2>\n<p><a href=\"#heading\" rel=\"noopener noreferrer nofollow\">Heading</a></p>")]
             public void TestToHtmlWithAutoLinks(string originalMd, string expectedHtml)
             {
                 _featureFlagService.Setup(x => x.IsMarkdigMdRenderingEnabled()).Returns(true);