fixed error with schema registry config in .net 461 (#774)

* fixed error with schema registry config in .net 461 (bad assumption on Convert.ToString

* remove net461 unit test project for now

* undoing accidental sln add

Author: mkellogg

src/Confluent.SchemaRegistry/CachedSchemaRegistryClient.cs

@@ -78,9 +78,8 @@ public CachedSchemaRegistryClient(IEnumerable<KeyValuePair<string, string>> conf
             try { this.identityMapCapacity = identityMapCapacityMaybe.Value == null ? DefaultMaxCachedSchemas : Convert.ToInt32(identityMapCapacityMaybe.Value); }
             catch (FormatException) { throw new ArgumentException($"Configured value for {SchemaRegistryConfig.PropertyNames.SchemaRegistryMaxCachedSchemas} must be an integer."); }
 
-            // Convert.ToString returns "" in the null case here.
-            var basicAuthSource = Convert.ToString(config.FirstOrDefault(prop => prop.Key.ToLower() == SchemaRegistryConfig.PropertyNames.SchemaRegistryBasicAuthCredentialsSource).Value);
-            var basicAuthInfo = Convert.ToString(config.FirstOrDefault(prop => prop.Key.ToLower() == SchemaRegistryConfig.PropertyNames.SchemaRegistryBasicAuthUserInfo).Value);
+            var basicAuthSource = Convert.ToString(config.FirstOrDefault(prop => prop.Key.ToLower() == SchemaRegistryConfig.PropertyNames.SchemaRegistryBasicAuthCredentialsSource).Value) ?? "";
+            var basicAuthInfo = Convert.ToString(config.FirstOrDefault(prop => prop.Key.ToLower() == SchemaRegistryConfig.PropertyNames.SchemaRegistryBasicAuthUserInfo).Value) ?? "";
 
             string username = null;
             string password = null;