Improve the method to check if the SD is present

[GrazianoCapelli]

The $FMQ,LST returns $FMA,LST,0 in the following cases:

• if the SD is present (inserted) and empty;
• if the SD is not present;

The two cases must be separated.:

• The first case must return $FMA,LST,0 (zero files present);
• The second case must return $FMA,LST (no list: error);