Apply suggestions from code review

Co-authored-by: Michael Nebel <michaelnebel@github.com>

Author: mbg

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependabotProxy.cs

@@ -97,7 +97,7 @@ public struct RegistryConfig
                     // The value of the environment variable should be a JSON array of objects, such as:
                     // [ { "type": "nuget_feed", "url": "https://nuget.pkg.github.com/org/index.json" } ]
                     var array = JsonConvert.DeserializeObject<List<RegistryConfig>>(registryURLs);
-                    if (array != null)
+                    if (array is not null)
                     {
                         foreach (RegistryConfig config in array)
                         {

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/NugetPackageRestorer.cs

@@ -267,15 +267,15 @@ private void RestoreProjects(IEnumerable<string> projects, HashSet<string>? conf
             // `nuget.config` files instead of the command-line arguments.
             HashSet<string>? sources = null;
 
-            if (this.dependabotProxy != null)
+            if (this.dependabotProxy is not null)
             {
                 // If the Dependabot proxy is configured, then our main goal is to make `dotnet` aware
                 // of the private registry feeds. However, since providing them as command-line arguments
                 // to `dotnet` ignores other feeds that may be configured, we also need to add the feeds
                 // we have discovered from analysing `nuget.config` files.
                 sources = configuredSources ?? new();
                 sources.Add(PublicNugetOrgFeed);
-                this.dependabotProxy?.RegistryURLs.ForEach(url => sources.Add(url));
+                this.dependabotProxy.RegistryURLs.ForEach(url => sources.Add(url));
             }
 
             var successCount = 0;