fix: fall back to settings.COMPREHENSIVE_THEME_DIRS correctly

There was a logical error in the compile_sass management command:
instead of falling back to settings.COMPREHENSIVE_THEME_DIRS when
--theme-dirs was *None or missing*, we only fell back to it when
--theme-dirs was *missing*.

This caused theme compilation to be skipped when COMREHENSIVE_THEME_DIRS
*is not set* in the environment, even though
settings.COMPREHENSIVE_THEME_DIRS *is set* in Django settings, which
is currently the case for edx.org.

openedx/core/djangoapps/theming/management/commands/compile_sass.py

@@ -73,8 +73,8 @@ def handle(self, *args, **options):
             {"lms": "lms", "cms": "cms", "studio": "cms"}[sys]
             for sys in options.get("system", ["lms", "cms"])
         )
-        theme_dirs = options.get("theme_dirs", settings.COMPREHENSIVE_THEME_DIRS) or []
-        themes_option = options.get("themes", [])  # '[]' means 'all'
+        theme_dirs = options.get("theme_dirs") or settings.COMPREHENSIVE_THEME_DIRS or []
+        themes_option = options.get("themes") or []  # '[]' means 'all'
         if not settings.ENABLE_COMPREHENSIVE_THEMING:
             compile_themes = False
             themes = []