MailboxesAPI.getMailbox and getMessage return wrappers around null instead of null

[dmccoystephenson]

MailboxesAPI.getMailbox(Player) and MailboxesAPI.getMessage(int) (src/main/java/dansplugins/mailboxes/externalapi/MailboxesAPI.java) unconditionally wrap the lookup result:

public M_Mailbox getMailbox(Player player) {
    return new M_Mailbox(persistentData.getMailbox(player));
}

public M_Message getMessage(int ID) {
    return new M_Message(persistentData.getMessage(ID));
}

PersistentData.getMailbox and PersistentData.getMessage both return null when nothing is found, so a non-null M_Mailbox / M_Message is handed back whose delegate field is null. Every accessor on those wrappers delegates without a guard, so the failure surfaces to the integrating plugin as a NullPointerException at an arbitrary later call site rather than as a null return at the lookup.

This makes the idiomatic guard shown in API.md ineffective — if (message != null) is always true. API.md has been updated to describe the current behavior accurately, but the behavior itself is worth reconsidering, because it is the cross-plugin integration contract.

Suggested resolution

null should be returned when the underlying lookup finds nothing, so that integrating plugins can guard the lookup. This is a breaking change for any caller that currently relies on a non-null return, so it should be paired with an API version bump (currently v0.0.3) and a CHANGELOG.md entry.

This issue body was drafted during a Gardener session (https://github.com/Stephenson-Software/gardener).