Fix ValueError on message_attribute default value

[danbernstein]

Description

I pulled the repo to use the recently added FIFO queue option. When I followed the README example, I received the following error when I did not explicitly set "message_attributes={}" in send_messages.

ValueError: Message attribute name SQSLargePayloadSize is reserved for use by SQS extended client.

Looks like it has to do with the default value being mutable.

Fixes # (issue)
I created an empty dictionary if the default parameter is empty, and this resolved the issue.

PR type

• Feature
• TechDebt
• Bugfix
• Other

Changelog updated

• Yes
• No

Breaking changes

• Yes
• No

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• tox

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• PR has been appropriately with a meaning tag