fix(firestore, web): only set long polling options if it has a value

[russellwheatley]

Description

• Only set webExperimentalLongPollingOptions if there is a value set by the user.
• This ought to fix our web firestore e2e tests. The tests were failing because we wrap in try/catch here. When we tried to initialise, it would throw an exception because webExperimentalLongPollingOptions was null. Then the firestore instance would be reinitialised without settings and that means we wouldn't have cache option set.
• e2e tests now work which is proof of fix as previously getting doc from cache was failing because initialising firestore without settings wasn't occurring properly. See example test failure that is now fixed from this PR: https://github.com/firebase/flutterfire/actions/runs/10774838129/job/29877877726?pr=13292#step:11:87
• fixed firestore web wasm, the env var:

bool.fromEnvironment('dart.library.ffi');

is no longer available causing test failures as integers were now doubles again. I have updated to double check html package is not available instead as it isn't compatible with wasm.

Related Issues

Closes 13293

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