fix: successive optional route parameters can now be empty

[glennsayers]

Closes #9261

In the scenario outlined in the linked issue, with 2 chained optional parameters, skipping the first optional parameter would result in a 404.
When the first optional parameter was detected as being missing, a buffer variable was incremented to allow the 2nd parameter to be correctly assigned. However this buffer was never reset (except in the case of a [...rest] parameter being present). This was triggering the if(buffered) return; check, resulting in a 404.

This change resets the buffer back to 0 in the appropriate circumstance.
The check verifies that the next parameter is not a [...rest] parameter. If it is, we don't reset the buffer in order to trigger the coalescing of remaining values into [...rest].
If it's not a rest parameter, and the next value is defined, then we should reset the buffer to prevent any data loss. This scenario is tested in the following existing test:

{
	route: '/[[slug1=doesntmatch]]/[[slug2=matches]]/[[slug3=doesntmatch]]/[...rest].json',
	path: '/foo/bar/baz.json',
	expected: { slug2: 'foo', rest: 'bar/baz' }
},

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[Rich-Harris]

amazing, thanks so much!