fix: define dummy values when Rust extension unavailable

Without defining rust_dicttoxml, escape_xml_py, and wrap_cdata_py
when ImportError occurs, the test module fails to import entirely,
resulting in "collected 0 items" instead of properly skipped tests.

Author: vinitkumar

tests/test_rust_dicttoxml.py

@@ -15,6 +15,10 @@
     RUST_AVAILABLE = True
 except ImportError:
     RUST_AVAILABLE = False
+    # Define dummy functions so tests can be collected (they'll be skipped anyway)
+    rust_dicttoxml = None  # type: ignore[assignment]
+    escape_xml_py = None  # type: ignore[assignment]
+    wrap_cdata_py = None  # type: ignore[assignment]
 
 import json2xml.dicttoxml_fast as fast_module
 from json2xml import dicttoxml as py_dicttoxml