ext/xml: Add an explicit test case about unsetting a handler with empty string

ext/xml/tests/set_handler_deprecation_empty_string.phpt

@@ -0,0 +1,25 @@
+--TEST--
+Unsetting a handler via an empty string should emit a deprecation
+--EXTENSIONS--
+xml
+--FILE--
+<?php
+
+/* Use xml_set_processing_instruction_handler() for generic implementation */
+function dummy() {}
+
+/* Create valid parser */
+$parser = xml_parser_create();
+xml_set_processing_instruction_handler($parser, dummy(...));
+
+try {
+    xml_set_processing_instruction_handler($parser, '');
+} catch (\Throwable $e) {
+    echo $e::class, ': ', $e->getMessage(), PHP_EOL;
+}
+
+?>
+END
+--EXPECTF--
+Deprecated: xml_set_processing_instruction_handler(): Passing non-callable strings is deprecated since 8.4 in %s on line %d
+END