MarkdownReader: don't raise AttributeError on empty files
MarkdownReader: don't raise AttributeError on empty files

Markdown.convert() returns early, without running any preprocessors, if
source.strip() is empty.

Before, Pelican would raise AttributeError in this case; now, it logs a
more friendly error:

ERROR: Skipping ./foo.md: could not find information about 'NameError: title'

which is more consistent with the error from empty .rst files:

ERROR: Skipping ./foo.rst: could not find information about 'NameError: date'

zackw/pelican,lucasplus/pelican,Scheirle/pelican,zackw/pelican,ingwinlu/pelican,avaris/pelican,farseerfc/pelican,getpelican/pelican,douglaskastle/pelican,douglaskastle/pelican,ingwinlu/pelican,avaris/pelican,ehashman/pelican,farseerfc/pelican,talha131/pelican,getpelican/pelican,Scheirle/pelican,iKevinY/pelican,zackw/pelican,iKevinY/pelican,douglaskastle/pelican,ehashman/pelican,Rogdham/pelican,Rogdham/pelican,Scheirle/pelican,talha131/pelican,ehashman/pelican,lucasplus/pelican,Rogdham/pelican,lucasplus/pelican