fix: simulate lambda behavior for empty payload

[russau]

When you invoke a function with no payload the Lambda service passes {} (an empty json object) to your handler input parameter. The LambdaTestTool currently sends a null parameter not matching the way the service works.

For example, the function below is configured with DefaultLambdaJsonSerializer. If you invoke with an empty payload in the LambdaTestTool the input parameter is null. Doing the the same in the Lambda service the input parameter is {} deserialized, which gives you an instance of APIGatewayProxyRequest with none of the properties set.

public async Task<APIGatewayProxyResponse> FunctionHandler(APIGatewayProxyRequest input, ILambdaContext context)

Description of changes:

• LambdaExecutor.cs - BuildParameters now passes {} for an empty payload
• InvokeFunctionTests.cs - WithEventParameterTest tests a method that takes an S3Event with an empty payload
• FunctionSignatureExamples.csproj - added reference to Amazon.Lambda.S3Events
• InstanceMethods.cs - added method that take an S3Event

[normj]

Merged into dev branch to go out in next release.

[normj]

This PR was released as part of version 0.11.2 of the test tool.