Add a test for previously-uncovered error condition in C14N (#14283)

ext/dom/tests/c14n_no_nodeset_returned.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Not returning a node set for C14N
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = new DOMDocument;
+$dom->loadXML('<root>test</root>');
+try {
+    $dom->documentElement->C14N(true, false, ['query' => 'string(./root/text())']);
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+XPath query did not return a nodeset