[displaymanager] Avoid unassigned variable

If displaymanagers is not set in the job config or globally,
it wasn't set at all, leading to a runtime error. Set to
None, so that the regular error message will be triggered.

FIXES #1002

src/modules/displaymanager/main.py

@@ -382,6 +382,7 @@ def run():
     If a displaymanager is in the list but not installed, a debugging message
     is printed and the entry ignored.
     """
+    displaymanagers = None
     if "displaymanagers" in libcalamares.job.configuration:
         displaymanagers = libcalamares.job.configuration["displaymanagers"]