Fix index error when message array is blank

This error occurs when running the gitlog2asciidoc.py
script on Gerrit from v2.2.1 to v2.2.2.1.

Change-Id: I7e9bca2f087765336fdb1f092c3b1453123d25f0

tools/gitlog2asciidoc.py

@@ -77,7 +77,7 @@ for line in stdout_value.splitlines(True):
     else:
         if line == '\n':
             # Don't add extra blank line if last one is already blank
-            if message[-1] != '+\n':
+            if message != [] and message[-1] != '+\n':
                 message.append('+\n')
         else:
             message.append(line)