Valore null in un loop javascript

[botmite]

Ciao a tutti, sono alle prese con un problemino che non riesco proprio a risolvere, spero in un vostro aiuto.

Con questo script effettuo un controllo su una serie di select con le stesse identiche opzioni per impedire ai miei utenti di inviare valori duplicati:

function checkDropdowns() {
 var iDropdowns = 5;
 var sValue;
 var sValue2;
 for(var i = 1; i <= iDropdowns; ++i) {
 sValue = document.getElementById('drop' + i).value;
 for(var j = 1; j <= iDropdowns; ++j) {
 sValue2 = document.getElementById('drop' + j).value;
 if ( i != j && sValue == sValue2 ) {
 alert('Hai selezionato due o più giocatori uguali!');
  return false;
   }
     }
      } 
    return true; }

e questo è il form:

<form  action="<?php echo $editFormAction; ?>" onsubmit="return(validate());" method="post" name="form1">              
 <table width="100%" border="0">                
        <tr>
 <td class="table1">Offerte</td>
</tr>
<tr>
<td>
 <select name="rc_player1" id="drop1">
  <option value="%">Seleziona</option>
  <?php do {  ?>
   <option value="<?php echo $row_comboplayer['id']?>"><?php echo $row_comboplayer['playerName']?> [<?php echo $row_comboplayer['playerOverall']; ?>] [<?php echo $row_comboplayer['playerLevel']; ?>]</option>
                    <?php
} while ($row_comboplayer = mysql_fetch_assoc($comboplayer));
  $rows = mysql_num_rows($comboplayer);
  if($rows > 0) {
      mysql_data_seek($comboplayer, 0);
	  $row_comboplayer = mysql_fetch_assoc($comboplayer);
  }
?>
   </select>
    </td>
    </tr>
<tr>
<td>
 <select name="rc_player2" id="drop2">
  <option value="%">Seleziona</option>
  <?php do {  ?>
   <option value="<?php echo $row_comboplayer['id']?>"><?php echo $row_comboplayer['playerName']?> [<?php echo $row_comboplayer['playerOverall']; ?>] [<?php echo $row_comboplayer['playerLevel']; ?>]</option>
                    <?php
} while ($row_comboplayer = mysql_fetch_assoc($comboplayer));
  $rows = mysql_num_rows($comboplayer);
  if($rows > 0) {
      mysql_data_seek($comboplayer, 0);
	  $row_comboplayer = mysql_fetch_assoc($comboplayer);
  }
?>
   </select>
    </td>
    </tr>
<tr>
<td>
 <select name="rc_player3" id="drop3">
  <option value="%">Seleziona</option>
  <?php do {  ?>
   <option value="<?php echo $row_comboplayer['id']?>"><?php echo $row_comboplayer['playerName']?> [<?php echo $row_comboplayer['playerOverall']; ?>] [<?php echo $row_comboplayer['playerLevel']; ?>]</option>
                    <?php
} while ($row_comboplayer = mysql_fetch_assoc($comboplayer));
  $rows = mysql_num_rows($comboplayer);
  if($rows > 0) {
      mysql_data_seek($comboplayer, 0);
	  $row_comboplayer = mysql_fetch_assoc($comboplayer);
  }
?>
   </select>
    </td>
    </tr>
<tr>
<td>
 <select name="rc_player4" id="drop4">
  <option value="%">Seleziona</option>
  <?php do {  ?>
   <option value="<?php echo $row_comboplayer['id']?>"><?php echo $row_comboplayer['playerName']?> [<?php echo $row_comboplayer['playerOverall']; ?>] [<?php echo $row_comboplayer['playerLevel']; ?>]</option>
                    <?php
} while ($row_comboplayer = mysql_fetch_assoc($comboplayer));
  $rows = mysql_num_rows($comboplayer);
  if($rows > 0) {
      mysql_data_seek($comboplayer, 0);
	  $row_comboplayer = mysql_fetch_assoc($comboplayer);
  }
?>
   </select>
    </td>
    </tr>
<tr>
<td>
 <select name="rc_player5" id="drop5">
  <option value="%">Seleziona</option>
  <?php do {  ?>
   <option value="<?php echo $row_comboplayer['id']?>"><?php echo $row_comboplayer['playerName']?> [<?php echo $row_comboplayer['playerOverall']; ?>] [<?php echo $row_comboplayer['playerLevel']; ?>]</option>
                    <?php
} while ($row_comboplayer = mysql_fetch_assoc($comboplayer));
  $rows = mysql_num_rows($comboplayer);
  if($rows > 0) {
      mysql_data_seek($comboplayer, 0);
	  $row_comboplayer = mysql_fetch_assoc($comboplayer);
  }
?>
   </select>
    </td>
    </tr>
<tr>
<td>
<input class="submitbutton" name="mysubmit" onclick="return checkDropdowns();" type="submit" value="Aggiungi Offerta" /></td>
</tr>
</table>
</form>

I miei utenti non sono obbligati ad effettuare tutte e 5 le selezioni e questo controllo funziona solo se in tutte le select viene selezionata un opzione.
Come posso modificare lo script ed evitare che il valore null (che mel mio caso è "%") venga considerato?