fix: pass null to ReflectionProperty::setValue() for static Assert::$count

[Kimtran-art]

Running browser tests on PHP 8.3+ emits, on essentially every assertion retry:

Calling ReflectionProperty::setValue() with a 1st argument which is not null or an object is deprecated

It comes from Execution::resetAssertions(). Assert::$count is a static property, so under PHP 8.3 the first argument to setValue() must be null (or an object); passing the class-string Assert::class triggers the deprecation. On Firefox nearly every test retries once, so runs get flooded with the notice (tests still pass, but every test is labelled deprecated).

Fix: pass null, the canonical form for a static property - no behaviour change. The @phpstan-ignore-next-line is dropped since the call is now valid.

The same line exists on 5.x and is affected identically, if you'd like it forward-ported.