Enable deleting all elements from a tree

[lukasdreyer]

It is currently not supported to delete all elements of a global tree. Some of the completely empty trees go missing during a partitioning step.

There is already a disabled test that fails, see t8_gtest_empty_global_tree

This problem probably occurs during the partition send step.

In t8_forest_partition_sendloop, we determine how many elements to send in total, and iterate over all procs that we need to send elements to. In t8_forest_partition_fill_buffer, we iterate over all local trees, until all elements have been sent.

In the case that there is a tree afterwards, which had all its elements deleted, this tree will not be sent, and is therefore lost.