Bugfix: The f_memory array append and append all need to allow for sources to be...

Bugfix: The f_memory array append and append all need to allow for sources to be NULL.

A valid array that is not allocated will have a size of 0.
Passing these to the function should not result in an error.
If the size is 0, then there is nothing to copy even though array is NULL
This is all fine.

Update the documentation comments to be more explicit on NULL in the parameters.