Fix CA1062 warning (#2217)

Fix for `ContextualTtl`.

Author: Zombach

src/Polly/Caching/ContextualTtl.cs

@@ -4,7 +4,6 @@ namespace Polly.Caching;
 /// <summary>
 /// Defines a ttl strategy which will cache items for a TimeSpan which may be influenced by data in the execution context.
 /// </summary>
-#pragma warning disable CA1062 // Validate arguments of public methods
 public class ContextualTtl : ITtlStrategy
 {
     /// <summary>
@@ -28,6 +27,11 @@ public class ContextualTtl : ITtlStrategy
     /// <returns>TimeSpan.</returns>
     public Ttl GetTtl(Context context, object? result)
     {
+        if (context is null)
+        {
+            throw new ArgumentNullException(nameof(context));
+        }
+
         if (!context.ContainsKey(TimeSpanKey))
         {
             return NoTtl;

test/Polly.Specs/Caching/ContextualTtlSpecs.cs

@@ -2,6 +2,14 @@
 
 public class ContextualTtlSpecs
 {
+    [Fact]
+    public void Should_throw_when_context_is_null()
+    {
+        Context context = null!;
+        Action action = () => new ContextualTtl().GetTtl(context, null);
+        action.Should().Throw<ArgumentNullException>().And.ParamName.Should().Be("context");
+    }
+
     [Fact]
     public void Should_return_zero_if_no_value_set_on_context() =>
         new ContextualTtl().GetTtl(new Context("someOperationKey"), null).Timespan.Should().Be(TimeSpan.Zero);