Only make VACOLS request in eager_load_legacy_appeals_for_tasks if necessary

[lowellrex]

AppealRepository.eager_load_legacy_appeals_for_tasks loads information from VACOLS for all legacy appeals associated with a batch of tasks in a single request (really 4 requests, but it only executes that code path once for the entire batch instead of once for each legacy appeal). This dramatically reduces the load times of task queues because now we only make a single request to VACOLS instead of many. However, even if there are no LegacyAppeals associated with the set of tasks we pass to this function we still make a request to VACOLS. This PR exists to return early from the function in this case to save us even more time.

Acceptance criteria

• Return early from AppealRepository.eager_load_legacy_appeals_for_tasks if there are no LegacyAppeals associated with the set of input tasks.

[lowellrex]

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Estimating this at 1. Possible 2 because "programming is hard" (Moore, "Zero to Code" pp. 311).