Fix /help command crash if a command doesn't have docstrings

This was caused by commit 7046c49, which escapes automatically
docstrings without HTML in them. The problem was, the check if the
docstring contained HTML was done even if there was no string, causing
issues because you can't do the checks on None.

Regression-of: 7046c49

Author: Pietro Albini

botogram/defaults.py

@@ -158,6 +158,10 @@ def no_commands_hook(self, bot, chat, message):
 
 def escape_html(text):
     """Escape a docstring"""
+    # You can't escape None, right?
+    if text is None:
+        return
+
     # The docstring is escaped only if it doesn't contain HTML markup
     if not syntaxes.is_html(text):
         return html.escape(text)