prova questo:
<script>
function sendform(idform)
{
if(document.forms[idform].text_name.value != "")
document.forms[idform].submit()
else
alert('vuoto')
}
</script>
<body >

<form name="form_name" action="javascript: alert('ho sottomesso il form')">
<input name="text_name" type="text" value="" onBlur="javascript:sendform('0')" >

</form>