Adds exception to allow metadataSource to be blank (#115)

* Adds exception to allow metadataSource to be blank

* Updates the exception to check for null or whitlespace to be more inclusive

src/Lidarr.Api.V1/Config/MetadataProviderConfigModule.cs

@@ -1,7 +1,10 @@
+using System;
 using System.Linq;
 using System.Reflection;
+using FluentValidation;
 using NzbDrone.Core.Configuration;
 using Lidarr.Http;
+using NzbDrone.Common.Extensions;
 using NzbDrone.Core.Validation;
 
 namespace Lidarr.Api.V1.Config
@@ -11,7 +14,7 @@ namespace Lidarr.Api.V1.Config
         public MetadataProviderConfigModule(IConfigService configService)
             : base(configService)
         {
-            SharedValidator.RuleFor(c => c.MetadataSource).IsValidUrl();
+            SharedValidator.RuleFor(c => c.MetadataSource).IsValidUrl().When(c => !c.MetadataSource.IsNullOrWhiteSpace());
         }
 
         protected override MetadataProviderConfigResource ToResource(IConfigService model)