fix: a machine with no ifupdown configuration is not an error

The release validator caught it: in a container there is no
/etc/network/interfaces at all, and a missing file was being treated as
a read failure, so --check printed nothing and said nothing was wrong
rather than saying there was nothing to do.

A machine with no ifupdown configuration is the ordinary case for a
fresh install, and the answer is simply that this command has no work.

Co-Authored-By: Claude Opus 5 <[email protected]>

scripts/classroom-wifi

@@ -208,6 +208,11 @@ def carrying(name, device):
 
 def find_stanza():
     """The one wireless stanza ifupdown holds, or None, plus the file."""
+    if not os.path.exists(INTERFACES):
+        # A machine with no ifupdown configuration at all. Nothing is
+        # wrong with that and nothing here has anything to do.
+        return None, ""
+
     try:
         with open(INTERFACES, "r", encoding="utf-8") as handle:
             text = handle.read()