fix for bug stopping vault timeout to never (#1618)

- use nullable int on settings page and in vault service

src/Core/Abstractions/IVaultTimeoutService.cs

@@ -17,6 +17,6 @@ namespace Bit.Core.Abstractions
         Task LockAsync(bool allowSoftLock = false, bool userInitiated = false);
         Task LogOutAsync();
         Task SetVaultTimeoutOptionsAsync(int? timeout, string action);
-        Task<int> GetVaultTimeout();
+        Task<int?> GetVaultTimeout();
     }
 }