'POST body missing' when programmatically invoking graphql handler in v3

When I programmatically invoke the graphql handler of the example posted by @Alec2435 in #5462, I get the following error:

{
    "body": "POST body missing, invalid Content-Type, or JSON object has no keys.", 
    "isBase64Encoded": false, 
    "multiValueHeaders":  {
        "access-control-allow-origin": ["*"], 
        "content-length": ["68"], 
        "content-type": ["text/html; charset=utf-8"], 
        "etag": ["W/\"44-LstwwwiSrcEeuZb+mu4TKVPIfxQ\""], 
        "x-powered-by": ["Express"]
    },
    "statusCode": 400
}

• The package name and version of Apollo showing the problem.

I’m using “apollo-server-lambda” version 3.0.1.

• The expected behavior.

That it executes as in v2.

• The actual behavior.

It returns the message “POST body missing, invalid Content-Type, or JSON object has no keys.”.

• A simple, runnable reproduction!

I’m executing this code:

   const event  = generateEvent({
            query: `
              query {
                hello
              }
            `,
        });
    const result = await graphqlHandler(event, {});
    expect(result.body).toEqual("{\"data\":{\"hello\":\"Hello world!\"}}\n");

Where the event is generated by:

exports.generateEvent = body => ({
    body: body ? JSON.stringify(body) : null,
    headers: {
        "Accept": "application/json",
        "Content-Type": "application/json",
    },
    multiValueHeaders: {},
    httpMethod: "POST",
    isBase64Encoded: false,
    path: "/",
    pathParameters: null,
    queryStringParameters: null,
    multiValueQueryStringParameters: null,
    stageVariables: null,
    requestContext: {
        accountId: "1234567890",
        apiId: "appid",
        httpMethod: "POST",
        identity: {},
        authorizer: {},
        protocol: "HTTP/1.1",
        path: "/",
        stage: "dev",
        requestId: "test",
        requestTimeEpoch: 0,
        resourceId: "none",
        resourcePath: "/",
    },
    resource: "/",
});

Is it a bug or am I doing something wrong? In v2 it works perfectly!

_Originally posted by @tiagocpeixoto in https://github.com/apollographql/apollo-server/issues/5462#issuecomment-881956725_