Fix GH-19480: error_log php.ini cannot be unset when open_basedir is configured

Since the ini message handlers already check for basedir, we need to
drop the basedir check from ini_set. Then we also fix the exceptional
case for the empty string: it should bypass the basedir check.
Furthermore, there was a regression introduced with the error_log
"syslog" check in ddfe269a (inverted check), so we fix that as well.

Closes GH-19487

tests/security/error_log_special_values.phpt

@@ -0,0 +1,13 @@
+--TEST--
+Setting error_log to special values with open_basedir enabled
+--INI--
+open_basedir=foo
+error_log=
+--FILE--
+<?php
+var_dump(ini_set("error_log", "syslog"));
+var_dump(ini_set("error_log", ""));
+?>
+--EXPECT--
+string(0) ""
+string(6) "syslog"