How do I check if a field is blank or not, using a selector?

[desbest]

How do I check if a field is blank or not, using a selector?

I couldn't find any information on how to do this on the Selectors documentation, and on the forum.

I would like to use $pages->find()

Do I have a NULL variable and do something like 

$nullvar = NULL;
$companiesacceptedawaitingemail = $pages->find("template=company, approved=1, approval_email_sent_datenumber!=$nullvar");

 

[abdus]

Checking for empty value is usually enough

$emptyPages = $pages->find("myField=''") // note the empty string with single quotes
$filledPages = $pages->find("myField!=''")