fix: added null check

Author: egil

src/bunit.web/Asserting/DiffAssertExtensions.cs

@@ -38,6 +38,9 @@ public static void ShouldHaveChanges(this IEnumerable<IDiff> diffs, params Actio
 		{
 			if (diffs is null)
 				throw new ArgumentNullException(nameof(diffs));
+			if (diffInspectors is null)
+				throw new ArgumentNullException(nameof(diffInspectors));
+
 			if (diffs.Take(diffInspectors.Length + 1).Count() != diffInspectors.Length) // Optimized way of writing "diffs.Count() != diffInspectors.Length"
 				throw new ActualExpectedAssertException(diffs.Count().ToString(CultureInfo.InvariantCulture), diffInspectors.Length.ToString(CultureInfo.InvariantCulture), "Actual changes", "Expected changes", "The actual number of changes does not match the expected.");