Minor CS improvements in NzbDroneValidation

(cherry picked from commit 6118afa339621509aad55caf27b05e89bd0b8c74)

Fixes #3714
Fixes #3715

src/NzbDrone.Core/Validation/NzbDroneValidationExtensions.cs

@@ -26,8 +26,7 @@ namespace NzbDrone.Core.Validation
         {
             foreach (var item in list)
             {
-                var extended = item as NzbDroneValidationFailure;
+                if (item is NzbDroneValidationFailure { IsWarning: true })
-                if (extended != null && extended.IsWarning)
                 {
                     continue;
                 }

src/NzbDrone.Core/Validation/NzbDroneValidationFailure.cs

@@ -24,7 +24,7 @@ namespace NzbDrone.Core.Validation
             CustomState = validationFailure.CustomState;
             var state = validationFailure.CustomState as NzbDroneValidationState;
 
-            IsWarning = state != null && state.IsWarning;
+            IsWarning = state is { IsWarning: true };
         }
     }
 }

src/NzbDrone.Core/Validation/NzbDroneValidationResult.cs

@@ -26,8 +26,7 @@ namespace NzbDrone.Core.Validation
 
             foreach (var failureBase in failures)
             {
-                var failure = failureBase as NzbDroneValidationFailure;
+                if (failureBase is not NzbDroneValidationFailure failure)
-                if (failure == null)
                 {
                     failure = new NzbDroneValidationFailure(failureBase);
                 }