Fixing logic in custom or HB helper

[teresaqhoang]

Motivation and Context

The custom or helper had a bug where it was checking for null values, rather than checking for non-null values. This PR changes the condition to not null.

Also, renamed a heading in the CreatePlan prompt to better guide the model in identifying Handlebars built-in block helpers.

Description

Additional changes to the GetArgumentValue method in KernelHelperUtils.cs to return null if variable isn't defined.

Before, if the template referenced a variable that wasn't defined in KernelArguments, the method would just return the key as a string. Now, it just returns null (as expected).

I had it returning a string before because I thought Handlebars would default the argument to its string representation since it templates regular text, but I double checked HB docs, and generally, string literals are surrounded by quotes when a raw value is passed as an argument.

Should resolve these issues:

• .Net: Get Method missing in Handlebars planner to parse JSON output #4661
• .Net: Include output scheme in functions manual for HandlebarsPlanner #4732

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄