[Github] Fix github automation script on empty descriptions

[boomanaiden154]

Before this patch, the github automation script would fail when trying to escape the text of a PR/issue description that was empty as the Github library returns None instead of an empty string in those scenarios. This patch special cases this and makes the escape function return an empty string when given a value of None.

[boomanaiden154]

This addresses failures like the one present in https://github.com/llvm/llvm-project/actions/runs/6753398365/job/18360167668:

Traceback (most recent call last):
  File "/home/runner/work/llvm-project/llvm-project/./github-automation.py", line 702, in <module>
    pr_subscriber.run()
  File "/home/runner/work/llvm-project/llvm-project/./github-automation.py", line 163, in run
    body = escape_description(self.pr.body)
  File "/home/runner/work/llvm-project/llvm-project/./github-automation.py", line 52, in escape_description
    str = html.escape(str, False)
  File "/usr/lib/python3.10/html/__init__.py", line 1[9](https://github.com/llvm/llvm-project/actions/runs/6753398365/job/18360167668#step:3:10), in escape
    s = s.replace("&", "&amp;") # Must be done first!
AttributeError: 'NoneType' object has no attribute 'replace'

Not sure if this is the most optimal way to handle it (might want to handle it outside of the escape function), but I'm happy to implement feedback from reviewers here.

[tstellar]

Thanks for fixing this.

[M4ximumPizza]

Fantastic fix. Thanks!

[boomanaiden154]

@M4ximumPizza Can you please stop reviewing random PRs unless you have something to add to the conversation? Thanks!