Preserve vanilla drop behavior when tile-drop flag is applied

[hochbaum]

Description

Currently, the tile-drop flag does not respect the initial value of BlockDestroyEvent#willDrop, which is set by the server before the event is fired. As a result, some blocks that would not drop in a vanilla environment end up dropping.

You can reproduce that by doing the following:

1. Go to a plot which has tile-drop set to true
2. Spawn a sniffer egg using /setblock ~ ~ ~ minecraft:sniffer_egg[hatch=2]
3. Wait until the egg hatches

You will notice that the egg block is dropped.

This PR fixes the issue by adding a simple check to ensure PlotSquared only sets willDrop when it is already true, preserving vanilla behavior.

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.

[PierreSchwang]

lgtm