Minor argument check fixes

Fixed `nameof()` reference to be consistent with arg being checked. Added null check for port to be consistent with earlier overload.

src/Middleware/HealthChecks/src/Builder/HealthCheckApplicationBuilderExtensions.cs

@@ -200,6 +200,11 @@ namespace Microsoft.AspNetCore.Builder
             }
 
             if (path == null)
+            {
+                throw new ArgumentNullException(nameof(path));
+            }
+
+            if (port == null)
             {
                 throw new ArgumentNullException(nameof(port));
             }