A supply nobody could read is unknown, not empty

plugins/printers/services/supply_parts.py

@@ -68,19 +68,34 @@ def derivecolor(name: str, tagcolor: Optional[str] = None) -> str:
     return "none"
 
 
-def classifysupply(level: float, name: str, vendor: Optional[str]) -> Dict:
-    """Classify one supply item into ok/low/critical.
+def classifysupply(level: Optional[float], name: str,
+                   vendor: Optional[str]) -> Dict:
+    """Classify one supply item into ok/low/critical, or unknown.
 
     Waste cartridge fill is inverted vs a toner level: a full waste cartridge
     is bad. Standard vendors report waste as percent FULL (high = bad). Xerox
     EC/AltaLink series report waste as percent capacity REMAINING (low = bad),
     same direction as toner. Normalise everything to percent remaining first.
+
+    A level of None means Zabbix has no usable reading; the status is 'unknown'
+    and remaining is None. Callers must not treat that as a low supply.
     """
     lowername = (name or "").lower()
     iswaste = "waste" in lowername
     isdrum = "drum" in lowername or "imaging" in lowername
     isxerox = bool(vendor) and "xerox" in vendor.lower()
 
+    # No reading is its own answer, not a level. It cannot be classified ok,
+    # low or critical without asserting something nobody measured - and every
+    # one of those three is a claim the reader would act on.
+    if level is None:
+        return {
+            "status": "unknown",
+            "remaining": None,
+            "iswaste": iswaste,
+            "isdrum": isdrum,
+        }
+
     if iswaste and not isxerox:
         remaining = 100 - level
     else: