fhem.pl: Do not regexp-check undef in ReadingsNum (Foum #70496)

git-svn-id: https://svn.fhem.de/fhem/trunk@13983 2b470e98-0d58-463d-a4d8-8e2adae1ed80

fhem/fhem.pl

@@ -3974,6 +3974,7 @@ ReadingsNum($$$;$)
 {
   my ($d,$n,$default,$round) = @_;
   my $val = ReadingsVal($d,$n,$default);
+  return undef if(!defined($val));
   $val = ($val =~ /(-?\d+(\.\d+)?)/ ? $1 : "");
   $val = round($val,$round) if($round);
   return $val;