Only one player can register using DATABASE strategy without emails

[aleokdev]

Describe the bug

When trying to register, a single message is sent, "Someone has already registered with this username or email", even though only one other user has registered (with a different username)

To Reproduce

Steps to reproduce the behavior:

1. Connect to the server, which has email requirement OFF
2. Register with one account
3. Try to register with another completely different one

Expected behavior

It should let me register.

Versions

Authmod 3.0, Minecraft 1.12.2

[Chocorean]

Working correctly with FILE strategy at least as of b015d88

[Chocorean]

Bug has been reproduced with database strategy. @Mcdostone can you check for 1.15.2 if this is working properly?

[Chocorean]

Issue identified:
Right now, using DATABASE strategy and without using emails, only one player can register because the email field is required and unique, but it is also equal to '' for everyone.
As it also compares by emails, the first player to register already has '' as email address, which results in a PlayerAlreadyExistException

[Chocorean]

@Mcdostone I think this is still happening on your branch