Parser: Allow empty attributes in default parsers

[dmsnell]

Since the introduction of the default parser in #8083 we have had a
subtle bug in the parsing which failed when empty attributes were
specified in a block's comment delimiter - {}

The absense of attributes was fine but empty attributes were a
failure. This is due to using +? in the RegExp tokenizer instead of
using *? (which allows for no inner content in the JSON string).

This patch updates the quantifier to restore functionality and fix the
bug. This didn't appear in practice because we don't intentionally set
{} as the attributes - the serializer drops it altogther, and our
tests didn't catch it for similar reasons.

Testing

Open up a post with the code editor.
Enter <!-- wp:test {} /--> and see if it breaks.
If it breaks, once you hit Save Draft it will replace it with <p><!-- wp:test {} /--></p>

If it's working, you won't see it change on Save Draft

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[mcsf]

Thanks for the quick fix! Still, even though our serialiser drops the empty object, can we add a test case?

[dmsnell]

@mcsf the test is in #11434 and it'd be much easier from a merge-standpoint to push this fix out independently of the tests. I can go through all the work of moving around code and managing the conflicts if it's important to you, but I'd rather see us merge both this and that one right next to each other - this is a tangent that could have been rolled into #11434 but I wanted to keep it separate for the sake of separating different changes.

if you want the test then maybe I'll close this PR and add the fix in #11434

what do you say?

[dmsnell]

Except now I'm confused why the tests in #11434 are passing since this is tested there…will do some more investigation

[dmsnell]

now I'm confused why the tests in #11434 are passing

haha, it was passing because <p><!-- wp:void {} /--></p> is a freeform block whose attributes are {}

[dmsnell]

Merging this fix into #11434