HardwareClientBase: provide default implementation for label

and add warning about placeholders

electrum/plugins/digitalbitbox/digitalbitbox.py

@@ -88,10 +88,6 @@ class DigitalBitbox_Client(HardwareClientBase):
         pass
 
 
-    def label(self):
-        return " "
-
-
     def is_pairable(self):
         return True
 

electrum/plugins/hw_wallet/plugin.py

@@ -196,7 +196,9 @@ class HardwareClientBase:
         and they are also used as a fallback to distinguish devices programmatically.
         So ideally, different devices would have different labels.
         """
-        raise NotImplementedError()
+        # When returning a constant here (i.e. not implementing the method in the way
+        # it is supposed to work), make sure the return value is in electrum.plugin.PLACEHOLDER_HW_CLIENT_LABELS
+        return " "
 
     def get_soft_device_id(self) -> Optional[str]:
         """An id-like string that is used to distinguish devices programmatically.

electrum/plugins/ledger/ledger.py

@@ -80,9 +80,6 @@ class Ledger_Client(HardwareClientBase):
     def is_initialized(self):
         return True
 
-    def label(self):
-        return ""
-
     def get_soft_device_id(self):
         if self._soft_device_id is None:
             # modern ledger can provide xpub without user interaction