Resolution for bug1269 - stop the hydrogen placer attempting to place hydrogens on null atoms.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>
Tidy up of HydrogenPlacer including test annotations and additional assertions about placed/unplaced atoms.

Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>