Add a new test for the sqs poller when message is empty

[sandbergja]

What maintenance needs to be done?

Test the behavior of the AwsSqsPoller class when the message body is an empty string.

Level of urgency

• High
• Moderate
• Low

Why is this maintenance needed?

We have been experiencing timeout errors related to the Poller (see #1566). We are not sure of the behavior of this class when it receives unexpected/empty data, and would like to see if it could be related to that issue.

Acceptance criteria

• the AwsSqsPoller tests include a test in which the message's body is an empty string