How do I check if a Sql server string is null or empty

I think this:

SELECT 
  ISNULL(NULLIF(listing.Offer_Text, ''), company.Offer_Text) AS Offer_Text
FROM ...

is the most elegant solution.

And to break it down a bit in pseudo code:

// a) NULLIF:
if (listing.Offer_Text == '')
  temp := null;
else
  temp := listing.Offer_Text; // may now be null or non-null, but not ''
// b) ISNULL:
if (temp is null)
  result := true;
else
  result := false;

SELECT
   CASE WHEN LEN(listing.OfferText) > 0 THEN listing.OfferText 
        ELSE COALESCE(Company.OfferText, '') END 
   AS Offer_Text,

... 

In this example, if listing.OfferText is NULL, the LEN() function should also return NULL, but that's still not > 0.

Update

I've learned some things in the 5 1/2 years since posting this, and do it much differently now:

COALESCE(NULLIF(listing.OfferText,''), Company.OfferText, '')

This is similar to the accepted answer, but it also has a fallback in case Company.OfferText is also null. None of the other current answers using NULLIF() also do this.

Select              
CASE
    WHEN listing.OfferText is null or listing.OfferText = '' THEN company.OfferText
    ELSE COALESCE(Company.OfferText, '')
END As Offer_Text,         
from tbl_directorylisting listing  
 Inner Join tbl_companymaster company            
  On listing.company_id= company.company_id