Conversion pipeline: Handle input documents with no text. Allows conversion of MOBI files tha are only a sequence of images. Fixes #5554 (cannot convert kindle (mobi) comics to another format)

src/calibre/ebooks/oeb/transforms/flatcss.py

@@ -164,7 +164,10 @@ class CSSFlattener(object):
             body = html.find(XHTML('body'))
             fsize = self.context.source.fbase
             self.baseline_node(body, stylizer, sizes, fsize)
+        try:
             sbase = max(sizes.items(), key=operator.itemgetter(1))[0]
+        except:
+            sbase = 12.0
         self.oeb.logger.info(
             "Source base font size is %0.05fpt" % sbase)
         return sbase