Issue #19815: Fix segfault when parsing empty namespace declaration.

Based on patches by Christian Heimes and Vajrasky Kok

diff --cc Lib/test/test_xml_etree.py
Simple merge

diff --cc Modules/_elementtree.c
index 46a1f4157cc7267fcc06cb6a5e8f46169d791473,f45893f15dd8ccc467973379c57d9c32a1ba9935..b3b69767086c2797cda702c7bec9688860c445be
--- 1/Modules/_elementtree.c
--- 2/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@@ -3035,10 -2997,10 +3035,13 @@@ expat_start_ns_handler(XMLParserObject
      PyObject* sprefix = NULL;
      PyObject* suri = NULL;
  
-     suri = PyUnicode_DecodeUTF8(uri, strlen(uri), "strict");
 +    if (PyErr_Occurred())
 +        return;
 +
+     if (uri)
+         suri = PyUnicode_DecodeUTF8(uri, strlen(uri), "strict");
+     else
+         suri = PyUnicode_FromString("");
      if (!suri)
          return;