EPUB Output: If there are no guide elements do not output an empty guide tag. Makes the asinine epubcheck happy. Fixes #1835560 [epub file fails at epub check](https://bugs.launchpad.net/calibre/+bug/1835560)

src/calibre/ebooks/oeb/base.py

@@ -1486,6 +1486,8 @@ class Guide(object):
         return elem
 
     def to_opf2(self, parent=None):
+        if not len(self):
+            return
         elem = element(parent, OPF('guide'))
         for ref in self.refs.values():
             attrib = {'type': ref.type, 'href': urlunquote(ref.href)}