Allow passing arguments to update_cubble()

[loreabad6]

Hi Sherry,

I was looking for a function where I could update the key and index of a cubble and bumped into update_cubble().
However, internally the key, index, etc. are preset to the original values from the data object. Adding a simple check for NULL values would allow to pass the arguments and in fact update the cubble. This would be mainly helpful for me since new_spatial_cubble() is not exported. I hope this doesn't break anything, I did not test much yet.