[Cashtab][Alias] Add trimmed input null check after input clearance

Summary:
T3363

UAT picked up an edge case where after inputting a valid alias input and then clearing it from the input field, the keyup event will trigger an API call on the-now null value.
This diff prevents the alias API check if the trimmed input value is null or an empty string.

Test Plan:
npm test
npm start
enter a valid alias, wait for 1 second, then delete it
ensure no "Error retrieving alias status" pops up on the UI
regression test the recent feature where entering a registered alias (e.g. watermelon) will render an input error on the frontend

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14918