Fixed minor bug in constants.processQuantity().
Fixed minor bug in constants.processQuantity().

The bug occurs when None is passed to processQuantity(); instead of returning
None, we now return (None, '') so that every exit point from
processQuantity() is a 2-tuple (value, units).

chatelak/RMG-Py,faribas/RMG-Py,comocheng/RMG-Py,nickvandewiele/RMG-Py,enochd/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py,nickvandewiele/RMG-Py,pierrelb/RMG-Py,KEHANG/RMG-Py,KEHANG/RMG-Py,enochd/RMG-Py,faribas/RMG-Py,nyee/RMG-Py,chatelak/RMG-Py,comocheng/RMG-Py