It is not possible to short the posts in the way that you want to sort them. In MySQL empty strings are less than non-empty strings. When sorting in ASC order they will always come first. If you use number as the data type the value would be 0 (zero) again lower than any post with a date.
There are really 2 choices
1) ensure that every post has a value
2) do 2 queries, first get all posts with a value and then get all posts with empty values.
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