Handle null text in first_paragraphs()

[DasSkelett]

Problem

This link to a modpack on beta throws a 500: https://beta.spacedock.info/pack/1/some%20stuff

   File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/../templates/layout.html", line 10, in top-level template code
     {%- block opengraph %}
   File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/../templates/mod_list.html", line 9, in block "opengraph"
     <meta property="og:description" content="{{ mod_list.description | first_paragraphs | bleach }}">
   File "/var/www/virtual/spacedock.info/htdocs/SpaceDock/KerbalStuff/common.py", line 46, in first_paragraphs
     return '\n\n'.join(PARAGRAPH_PATTERN.split(text)[0:3])
 TypeError: expected string or bytes-like object

#385 made use of our first_paragraphs() function to get a short-ish text that can be put into tne og:description tag for mod packs.
However, the description column is nullable, and while it appears that new mod packs will always be initialized with an empty string, there can be old ones with NULL/None.

Pattern.split can only take strings as argument, and thus throws for None.

It would have been nice if mypy caught that, because first_paragraphs(test: str) explicitly asks for an argument that is not None, however as we already found out, it can't check Jinja templates. Additionally it seems to assume that ModList.description cannot be None.

Changes

first_paragraphs now accepts Optional[str] and returns an empty string if text is None.

This solution should be more future-proof than e.g. checking for None within the template before calling first_paragraph, as it catches all occurrences.

[HebaruSan]

Yup, this is exactly what I had in mind for this fix. 👍

[DasSkelett]

Whoops, now I see the typo on the commit message as well. Going to reword it later and merge afterwards