fhem.pl: avoid WARNING in FileRead if the argument is undefined

git-svn-id: https://svn.fhem.de/fhem/trunk@30311 2b470e98-0d58-463d-a4d8-8e2adae1ed80

fhem/fhem.pl

@@ -5783,6 +5783,7 @@ FileRead($)
       close($FH);
       chomp(@ret);
     } else {
+      $fileName = "'undefined'" if(!defined($fileName));
       $err = "Can't open $fileName: $!";
     }