fix: support accessing task from test context without accessing fixtures

[dsyddall]

Description

Fixes #4418

Currently, when extending a test function with fixtures, if the fixtures are unused within a given test or hook, when accessing non-fixture context (e.g. task) the test fails with an error:
TypeError: Cannot destructure property 'isFn' of 'fixture' as it is undefined.

Example:

const myTest = test.extend({ a: 1 })

myTest('accessing non-fixture context', ({ task }) => {
  expect(task).toBeTruthy()
})

This PR addresses the above error by making withFixtures return early if the pendingFixtures array is empty after having been filtered.

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