[MERGE] trml2pdf: avoid crashing on missing images in RML w/ reportlab 2.5
[MERGE] trml2pdf: avoid crashing on missing images in RML w/ reportlab 2.5

Image values that evaluate to False were not causing issues
with previous reportlab versions, but as of version 2.5 it does not work
anymore, hence this little fix.
xrg/openerp-server,xrg/openerp-server