Preprints author search error when multiple users have orcid=""

Describe the bug
A user clicking "search" without entering text on the add authors step of submitting a preprint produces an error: Account.MultipleObjectsReturned -- get() returned more than one Account -- it returned 2!

This only happens when there are 2 or more accounts where orcid = "" (rather than None)

Alainna believes that both of the accounts in this situation were added through the janeway interface: 1 during submission and 1 through registration.

Janeway version
1.5.0

To Reproduce

1. Add two users with orcid = ""
2. Start a preprint submission, click through to add author step
3. Don't enter any text in the search box
4. Click "search"

Expected behavior
Both or neither accounts should be returned. No error. :D

For the account model:

• we should ensure ORCIDs are unique to avoid MultipleObjectsReturned.
• Write a migration to handle potential duplicates with the following requirements:
  • If a dupe is found, make sure orcid is copied to any linked FrozenAuthor records before wiping the dupe
  • If a duplicate is found where an account has never logged in, it is safe remove the orcid value from it
  • If a duplicate is found where all accounts have logged in, wipe the orcid from all but the latest to have authenticated.
• The orcid field on the model should not be set as null=True.
  For the frozen author Models across site types:
• The orcid field on the model should not be set as null=True.

Finally, we should tweak the search so that we don't call get(orcid="") with a blank string

Start by tracing the preprint submission add-author search described in the report, then inspect the Account model and the FrozenAuthor models across site types. Review the migration requirements for duplicate blank ORCIDs and the blank-string search path; done means duplicate data is handled safely, model nullability is corrected, and searching without text no longer raises MultipleObjectsReturned.