Add support for the unit struct in Properties derive

[Xavientois]

Description

Added support for unit structs for deriving the Properties trait. Now, the following works:

#[derive(Properties, Clone, Default)]
struct MyUnitStruct;

Fixes #1677

Checklist

• I have run cargo make pr-flow
• I have reviewed my own code
• I have added tests

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #1677: Properties derive macro fails on empty struct

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[Xavientois]

When I ran cargo make pr-flow locally, the tests failed for html_macro. They also appear to be failing here. Is there anything I should do about this or should I just let it be, as it is unrelated to my PR?

@teymour-aldridge is this test being handled by #1748 ?

[teymour-aldridge]

I think this is an issue from the tests not being updated in an earlier PR (but I haven't traced down to see which one). The actual output looks to be correct (minus line numbers) so if you run with the environment variable TRYBUILD=overwrite set and then push the changes that should get the unit tests working.

Once this merges, I'll rebase my PR (I'm still trying to collect a little feedback on that one).

[siku2]

Went ahead and merged this so we can tackle the broken tests in #1748.
Thanks, @Xavientois!