Update unit test and use C# 9 feature for null checking.

Author: bchavez

Source/Bogus.Tests/StrictModeTests.cs

@@ -1,5 +1,4 @@
 using System;
-using Bogus.Tests.Models;
 using FluentAssertions;
 using Xunit;
 
@@ -97,13 +96,20 @@ public void cannot_use_rules_with_strictmode_inside_rulesets()
       }
 
       [Fact]
-      public void StrictMode_True_NoRules()
+      public void strictmode_with_no_rules_should_throw()
       {
-         Assert.Throws<ValidationException>(() => 
-         {          
-            new Faker<Order>()
-               .StrictMode(true).Generate(1);
-         });
+         var faker = new Faker<Examples.Order>()
+            .StrictMode(true);
+
+         Action act = () => faker.Generate(1);
+
+         act.Should().ThrowExactly<ValidationException>()
+            .WithMessage("*Missing Rules*")
+            .WithMessage("*Validation was called to ensure all properties*")
+            .WithMessage($"*{nameof(Examples.Order.OrderId)}*")
+            .WithMessage($"*{nameof(Examples.Order.Item)}*")
+            .WithMessage($"*{nameof(Examples.Order.Quantity)}*")
+            .WithMessage($"*{nameof(Examples.Order.LotNumber)}*");
       }
    }
 }

Source/Bogus/Faker[T].cs

@@ -744,7 +744,7 @@ private ValidationResult ValidateInternal(string[] ruleSets)
             {
                foreach( var propOrFieldOfT in userSet )
                {
-                  if( populateActions != null && populateActions.TryGetValue(propOrFieldOfT, out var populateAction) )
+                  if( populateActions is not null && populateActions.TryGetValue(propOrFieldOfT, out var populateAction) )
                   {
                      // Very much a .Rules() action
                      if( populateAction.ProhibitInStrictMode )