Register delegate: Erasing name input sends empty username availability request

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Steps to reproduce

1. Register delegate
2. Enter name
3. Erase name

Actual result

"...checking availablility"
request with empty username ->
400 bad response
{"message":"Validation errors","errors":[{"code":"INVALID_REQUEST_PARAMETER","message":"Invalid parameter (username): Value failed JSON Schema validation","in":"query","name":"username","errors":[{"code":"MIN_LENGTH","message":"String is too short (0 chars), minimum 1","path":[]}]}]}

Expected result

No request sent for empty string