attrmap.py: allow empty _attrmap attributes
attrmap.py: allow empty _attrmap attributes

kanarelo/reportlab,Distrotech/reportlab,kanarelo/reportlab,kanarelo/reportlab,Distrotech/reportlab,Distrotech/reportlab,Distrotech/reportlab,Distrotech/reportlab,kanarelo/reportlab,kanarelo/reportlab