[.NET][Templating] Whenever a null value is evaluated, a warning should be raised

Platform

• .NET WPF

Version of SDK

Templating nuget

Details

With the fix of #4574 an error was unearthed where expressions that are evaluated to null will provide a confusing user and developer experience as they won't know what may be failing.

For example, this test should fail or raise a warning but it doesn't

[TestMethod]
        public void TestExceptionHandling()
        {
            // ${LineItem} doesn't exist in the data context provided
            string jsonTemplate = @"{
            ""type"": ""AdaptiveCard"",
            ""body"": [
              {
                ""type"": ""Container"",
                ""items"": [
                  {
                    ""$data"": ""${LineItem}"",
                    ""type"": ""TextBlock"",
                    ""text"": ""${Milage}""
                  }
                ]
              }
            ]
        }";

            string jsonData = @"{
              ""LineItems"": [
                {
                    ""Milage"": 10
                },
                {
                    ""Milage"": 0
                }
              ]
            }";

            AdaptiveCardTemplate transformer = new AdaptiveCardTemplate(jsonTemplate);
            var context = new EvaluationContext
            {
                Root = jsonData
            };

            try
            {
                string cardJson = transformer.Expand(context);
                Assert.Fail("There should be an exception");
            }
            catch (AdaptiveTemplateException e)
            {
                Assert.AreEqual(@"'${LineItem}' at line, '8' is malformed for '$data : ' pair", e.Message);
            }
            catch
            {
                Assert.Fail();
                throw;
            }
        }

Start with AdaptiveCardTemplate.Expand and the TestExceptionHandling example in the issue, then trace how expressions such as ${LineItem} are evaluated against EvaluationContext. Add coverage for a missing or null evaluation and verify that the resulting behavior reports the problem rather than silently continuing.