Clean up final analyzer findings

Root cause: the latest push triggered one more static-analysis pass that flagged a disposable test-helper pattern, two Yahoo immutability nits, one unreachable Yahoo throw, and avoidable empty-array allocations in MapQuest.

Author: niemyjski

src/Geocoding.MapQuest/MapQuestGeocoder.cs

@@ -56,13 +56,13 @@ private IEnumerable<Address> HandleSingleResponse(MapQuestResponse res)
                                         select l);
         }
         else
-            return new Address[0];
+            return Array.Empty<Address>();
     }
 
     private IEnumerable<Address> HandleSingleResponse(IEnumerable<MapQuestLocation> locs)
     {
         if (locs is null)
-            return new Address[0];
+            return Array.Empty<Address>();
         else
         {
             return from l in locs.OfType<MapQuestLocation>()

src/Geocoding.Yahoo/OAuthBase.cs

@@ -30,8 +30,8 @@ public enum SignatureTypes
     /// </summary>
     protected class QueryParameter
     {
-        private string _name = null!;
-        private string _value = null!;
+        private readonly string _name = null!;
+        private readonly string _value = null!;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="QueryParameter"/> class.

src/Geocoding.Yahoo/YahooGeocoder.cs

@@ -128,8 +128,6 @@ private async Task<IEnumerable<YahooAddress>> ProcessRequest(HttpRequestMessage
                 {
                     throw new YahooGeocodingException(message, ex);
                 }
-
-                throw new YahooGeocodingException(message, new HttpRequestException(message));
             }
 
             using (var stream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false))

test/Geocoding.Tests/TestHttpMessageHandler.cs

@@ -20,14 +20,10 @@ protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage reques
 
     public static Task<HttpResponseMessage> CreateResponseAsync(HttpStatusCode statusCode, string? reasonPhrase = null, string? body = null)
     {
-        var response = new HttpResponseMessage(statusCode)
+        return Task.FromResult(new HttpResponseMessage(statusCode)
         {
-            ReasonPhrase = reasonPhrase
-        };
-
-        if (!String.IsNullOrWhiteSpace(body))
-            response.Content = new StringContent(body, Encoding.UTF8, "text/plain");
-
-        return Task.FromResult(response);
+            ReasonPhrase = reasonPhrase,
+            Content = String.IsNullOrWhiteSpace(body) ? null : new StringContent(body, Encoding.UTF8, "text/plain")
+        });
     }
 }