"attributes" prop should be optional in ServerSideRender

[chrisvanpatten]

Describe the bug
If you try to invoke ServerSideRender without an attributes prop, you get an error.

To Reproduce
Steps to reproduce the behavior:

1. Create a block a block without attributes, rendered via a ServerSideRender component, like so: <ServerSideRender block="my/block" />
2. See error: "Error loading block: Invalid parameter(s): attributes"

Expected behavior
If attributes isn't provided, it should be ignored inside ServerSideRender.

Workaround
It is possible to work around this by passing an empty value into attributes, such as <ServerSideRender block="my/block" attributes={{}} /> (and then obviously just ignore it in your render_callback).