Fix detection of empty mount point column for encrypted file systems (#775475)

create_row() populates the values for each row to be displayed in the UI
from the relevant Partition object.  However load_partitions(5 params)
independently decided if the Name, Mount Point and Label columns were
empty and should be displayed.

Getting the mount point value is more complex for encrypted file systems
because it has to call get_mountpoints() on the inner encrypted
Partition object.  load_partitions(5 params) didn't account for this.

Fix by making create_row() both copy the values into each row and at the
same time check if they are empty to decide if they should be displayed
or not.

Bug 775475 - Mount Point column displayed for encrypted file systems
             even when empty