Fix for bug #20471 (https://gna.org/bugs/?20471).
The problem was that the data pipe is_empty() method was not handling structural data correctly. The is_empty() method now belongs to the MolContainer objects rather than cdp.structure, and the object cdp.structure is not always present.
Authored by: bugman 2013-02-01
Parent: [r18358]
Child: [r18360]
