explicitly cast null string to empty string for PHP 8.1

lib/phast/server/System.inc.php

@@ -416,6 +416,9 @@
 				}
 			}
 
+			if ($torepl_nontenanted === null) {
+				$torepl_nontenanted = "";
+			}
 			$retval_nontenanted = str_replace("~", $torepl_nontenanted, $path);
 			$physicalFilePath = System::GetApplicationPath() . $retval_nontenanted;
 			
@@ -425,6 +428,10 @@
 			}
 			else
 			{
+				if ($torepl === null)
+				{
+					$torepl = "";
+				}
 				$retval = str_replace("~~", $torepl_nontenanted, $path);
 				$retval = str_replace("~", $torepl, $retval);
 			}