Mark (commonly used) Java methods NotNull/Nullable

[madsboddum]

Return values and method arguments for Java code. Cover common classes such as Player and CreatureObject, so we can reduce the likelihood of NullPointerExceptions.
Player#getCreatureObject
Player#getPlayerObject

This will also help with the Kotlin conversion somewhat, as we're required to be explicit about nullability there.

These changes alone will result in a big diff, so definitely try to keep the changes simple.

[madsboddum]

There's a lot of code that assumes Player#getCreatureObject and Player#getPlayerObject won't return null.

And if we did add checks for null in hundreds of methods, I suspect they would all have the same handling of the situation: Nothing should happen.

Perhaps a stronger separation between the zone server and the login server is a better alternative. The reason this can even happen is because the same object, Player, is used at both the login server and the zone server. When at the character selection screen, Player#getCreatureObject obviously makes no sense yet.

Slicing zone server specific stuff into a different object, like a ZonedCharacter, might solve this in a more elegant manner.
Player can return a null ZonedCharacter, but ZonedCharacter#getCreatureObject shouldn't ever return null.

When executing a command, we can have command handlers work on ZonedCharacter instead of Player. You can't execute commands from the login screen, so moving the command handlers etc. to the new class seems sensible.
Some command handlers may need information from Player, so ZonedCharacter can simply have a getPlayer method.