CSHARP-5404: InvalidOperationException due to null casting in ConnectionId.ServerValue (#1538)

Author: BorisDog

src/MongoDB.Driver/Core/Connections/ConnectionId.cs

@@ -106,7 +106,7 @@ public long LongLocalValue
         [Obsolete("Use LongServerValue instead.")]
         public int? ServerValue
         {
-            get { return (int)_serverValue; }
+            get { return (int?)_serverValue; }
         }
 
         /// <summary>

tests/MongoDB.Driver.Tests/Core/Connections/ConnectionIdTests.cs

@@ -92,6 +92,16 @@ public void LongLocalValue_should_be_what_was_specified_in_the_constructor(long
             subject.LongLocalValue.Should().Be(localValue);
         }
 
+        [Fact]
+        public void ServerValue_should_return_null_when_null()
+        {
+            var subject = new ConnectionId(__serverId, 10);
+
+#pragma warning disable CS0618 // Type or member is obsolete
+            subject.ServerValue.ShouldBeEquivalentTo(null);
+#pragma warning restore CS0618 // Type or member is obsolete
+        }
+
         [Theory]
         [InlineData(0)]
         [InlineData(int.MaxValue)]