Fix NPE when retrieving an entity with a null UUID

While the null UUID is almost certainly an error, the old
implementation did not NPE as it used a plain HashMap for lookup
by UUID, whereas we use a ConcurrentHashMap which will NPE on
null keys.

patches/server/Chunk-System-Starlight-from-Moonrise.patch

@@ -5994,7 +5994,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +
 +    @Override
 +    public Entity get(final UUID id) {
-+        return maskNonAccessible(this.entityByUUID.get(id));
++        return maskNonAccessible(id == null ? null : this.entityByUUID.get(id));
 +    }
 +
 +    public boolean hasEntity(final UUID uuid) {
@@ -6002,7 +6002,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +    }
 +
 +    public String getDebugInfo() {
-+        return "count_id:" + this.entityById.size() + ",count_uuid:" + this.entityByUUID.size() + ",region_count:" + this.regions.size();
++        return "count_id:" + this.entityById.size() + ",count_uuid:" + this.entityByUUID.size() + ",count_accessible:" + this.getEntityCount() + ",region_count:" + this.regions.size();
 +    }
 +
 +    protected static final class ArrayIterable<T> implements Iterable<T> {