adapt behavior of cups printer to match other implementations

src/escpos/printer/cups.py

@@ -205,9 +205,9 @@ class CupsPrinter(Escpos):
         """
         printer = self.printers.get(self.printer_name, {})
         state = printer.get("printer-state")
-        if not state:
-            return b""
-        return bytes((state,))
+        if not state or state in [4, 5]:
+            return b"8"  # offline
+        return b"0"  # online
 
     def close(self) -> None:
         """Close CUPS connection.

test/test_printers/test_printer_cups.py

@@ -170,7 +170,7 @@ def test_read_no_device(cupsprinter) -> None:
     """
     GIVEN a cups printer object
     WHEN device is None
-    THEN check the return value is b''
+    THEN check the return value is b'8'
     """
     cupsprinter.device = None
-    assert cupsprinter._read() == b""
+    assert cupsprinter._read() == b"8"