[FIX] runbot_merge: dashboard image is branch created via SQL

If a branch is created via SQL directly, one might forget to set its
`write_date`, leading the computation of the `last_modified` to be
*really* unhappy as it's asked to compare a bunch of datetime objects
to `False` in order to compute the `max`.

Substitute missing `write_date` with `datetime.min` so they are
extremely unlikely to influence the computation until and unless the
branch gets updated.

runbot_merge/controllers/dashboard.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 import base64
 import collections
 import colorsys
+import datetime
 import hashlib
 import io
 import json
@@ -160,7 +161,7 @@ def raster_render(pr):
     # last-modified should be in RFC2822 format, which is what
     # email.utils.formatdate does (sadly takes a timestamp but...)
     last_modified = formatdate(max((
-        o.write_date
+        o.write_date or datetime.datetime.min
         for o in chain(
             project,
             repos,