Fix PHP 8.1 strlen breakage (#713)

* Fix PHP 8.1 strlen breakage

This adds a fallback to an empty string if the argument is `null` for some reason.

* Update StreamConnection.php

* use `strval()`

Co-authored-by: Till Krüss <tillkruss@users.noreply.github.com>

src/Connection/StreamConnection.php

@@ -387,7 +387,7 @@ class StreamConnection extends AbstractConnection
         $buffer = "*{$reqlen}\r\n\${$cmdlen}\r\n{$commandID}\r\n";
 
         foreach ($arguments as $argument) {
-            $arglen = strlen($argument);
+            $arglen = strlen(strval($argument));
             $buffer .= "\${$arglen}\r\n{$argument}\r\n";
         }