help: Don't show empty Summary or Description sections

Signed-off-by: Shawn O. Pearce <sop@google.com>
(cherry picked from commit 60e679209a5495393ef584efaaad287fc8b77c51)
diff --git a/subcmds/help.py b/subcmds/help.py
index c5979fd..90b817d 100644
--- a/subcmds/help.py
+++ b/subcmds/help.py
@@ -94,6 +94,8 @@
           body = getattr(cmd, bodyAttr)
         except AttributeError:
           return
+        if body == '' or body is None:
+          return
 
         self.nl()