String response for lambda function - no need to stringify string

[vladaman]

What does this PR do?

Lambda string response doesn't need to be stringified. This is especially important when we want to respond to services such as Congito, which require JSON output.

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

Added type check for string:

body: response === null ? null : (typeof response === 'string' ? response : JSON.stringify(response)),

[Jarred-Sumner]

Thank you