Replaced Element boolean value by testing its length

Hanldes deprecation of ElementTree.Element.__bool__

pelix/rsa/edef.py

@@ -250,7 +250,7 @@ def _indent(self, element: ElementTree.Element, level: int = 0, prefix: str = "\
         """
         element_prefix = "\n{0}".format(level * prefix)
 
-        if element:
+        if len(element) != 0:
             if not element.text or not element.text.strip():
                 element.text = element_prefix + prefix