fix: Test xml.etree.ElementTree.Element truth value by 'is not None'

* In Python 3.14 testing the truth value of an xml.etree.ElementTree.Element is
  deprecated and will raise an exception. As of Python 3.12 this behavior will
  raise a DeprecationWarning:

  ```
  DeprecationWarning: Testing an element's truth value will raise an exception
  in future versions.  Use specific 'len(elem)' or 'elem is not None' test
  instead.
  ```

  To avoid this, determine the truth element by using the 'elem is not None'
  method.

src/pyhf/writexml.py

@@ -56,7 +56,7 @@ def _export_root_histogram(hist_name, data):
 # https://stackoverflow.com/a/4590052
 def indent(elem, level=0):
     i = "\n" + level * "  "
-    if elem:
+    if elem is not None:
         if not elem.text or not elem.text.strip():
             elem.text = i + "  "
         if not elem.tail or not elem.tail.strip():

tests/test_export.py

@@ -447,7 +447,7 @@ def test_integer_data(datadir, mocker):
     mocker.patch("pyhf.writexml._ROOT_DATA_FILE")
 
     channel = pyhf.writexml.build_channel(spec, channel_spec, {})
-    assert channel
+    assert channel is not None
 
 
 @pytest.mark.parametrize(