Duff, Welcome to the Forums.
The best way is to upgrade to Lr 2.1 and use the 'No Keywords' collection.
In Lr 1.4.1 , in the Left Hand Find panel, under Keywords, there's an 'Are Empty' category. I don't have 1.3 installed, so I don't know if that's available in that version.
If not, here's a work-around, in the Find panel, select Text 'Keywords', with Rule, 'Doesn't Contain', and in the target, enter the string < a e i o u > (without the brackets, and with spaces between). Unless you've been shooting in Kryszygistan, that should find (most of) your blank keywords.
As an aside, there's really no reason not to upgrade to 1.4.1, it's stable, and has some additional features. You can find that here:
LR updates, Windows. Although, you don't say whether you're Mac/Win.