Fix test that is failing on 3.4 sharded clusters.

Author: rstam

tests/MongoDB.Driver.Core.Tests/Core/Operations/DropCollectionOperationTests.cs

@@ -136,7 +136,10 @@ public void Execute_should_return_expected_result(
             var result = ExecuteOperation(subject, async);
 
             result["ok"].ToBoolean().Should().BeTrue();
-            result["ns"].ToString().Should().Be(_collectionNamespace.FullName);
+            if (result.Contains("ns"))
+            {
+                result["ns"].ToString().Should().Be(_collectionNamespace.FullName);
+            }
         }
 
         [SkippableTheory]