C#: Address review comments related to logging.

Author: michaelnebel

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

@@ -97,13 +97,19 @@ private DependabotProxy(IDependabotProxyConfiguration config, ILogger logger, Te
                         {
                             if (string.IsNullOrWhiteSpace(registry.Url))
                             {
-                                logger.LogDebug("Ignoring registry with empty URL.");
+                                logger.LogError("Ignoring registry with empty URL.");
+                                continue;
+                            }
+
+                            if (string.IsNullOrWhiteSpace(registry.Type))
+                            {
+                                logger.LogError($"Ignoring registry at '{registry.Url}' since it has no type.");
                                 continue;
                             }
 
                             // The array contains all configured private registries, not just ones for C#.
                             // We ignore the non-C# ones here.
-                            if (registry.Type is null || !registry.Type.Equals("nuget_feed"))
+                            if (!registry.Type.Equals("nuget_feed"))
                             {
                                 logger.LogDebug($"Ignoring registry at '{registry.Url}' since it is not of type 'nuget_feed'.");
                                 continue;

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

@@ -359,6 +359,10 @@ private List<string> GetReachableFallbackNugetFeeds()
                     logger.LogInfo($"Using NuGet feeds from nuget.config files as fallback feeds: {string.Join(", ", ExplicitFeeds.OrderBy(f => f))}");
                 }
             }
+            else
+            {
+                logger.LogInfo($"Using fallback NuGet feeds from environment variable '{EnvironmentVariableNames.FallbackNugetFeeds}'.");
+            }
 
             return GetReachableNuGetFeeds(fallbackFeeds, isFallback: true);
         }