Assign New Database Id

stsadm -o addcontentdb -url http://portal -databasename WSS_Content_DB -assignnewdatabaseid

content database shows zero site collections in it You probably need to update the site map in the config db.

$db = get-spcontentdatabase -Identity [insert guid of cdb here]
$db.RefreshSitesInConfigurationDatabase()

$app = get-spwebapplication –identity “url of the problem web app”
$app.contentdatabses.count

$db = get-spwebapplication –identity “url of the webapplication that had the null entry” 
$db.contentdatabses.delete(“guidOfProblemDb”)

$db = Get-SPDatabase -Identity "guidOfProblemDb"
$db.Delete()