Guard against null _gitHubClient (which happens during unit testing)

Author: Jericho

src/GitReleaseManager.Core/Provider/GitHubProvider.cs

@@ -85,7 +85,11 @@ public GitHubProvider(IGitHubClient gitHubClient, IMapper mapper)
             _mapper = mapper;
 
             var graphQLClient = new GraphQLHttpClient(new GraphQLHttpClientOptions { EndPoint = new Uri("https://api.github.com/graphql") }, new SystemTextJsonSerializer());
-            graphQLClient.HttpClient.DefaultRequestHeaders.Add("Authorization", $"bearer {_gitHubClient.Connection.Credentials.Password}");
+            if (!string.IsNullOrEmpty(_gitHubClient?.Connection?.Credentials?.Password))
+            {
+                graphQLClient.HttpClient.DefaultRequestHeaders.Add("Authorization", $"bearer {_gitHubClient.Connection.Credentials.Password}");
+            }
+
             _graphQLClient = graphQLClient;
         }