Account for empty content in caption

This is mainly to preserve test integrity

Author: facelessuser

pymdownx/blocks/caption.py

@@ -59,7 +59,8 @@ def update_tag(el, fig_type, fig_num, template, prepend):
                     p = children[0]
                     span = etree.Element('span', {'class': 'caption-prefix'})
                     span.text = value
-                    span.tail = (' ' + p.text) if p.text is not None else p.text
+                    empty = not bool(p.text)
+                    span.tail = (' ' + p.text) if not empty else p.text
                     p.text = None
                     p.insert(0, span)