Can i have a button appear if a field is blank .... and disappear if the field contains some thing.

Sorta. (FileMaker's common answer!)

You can store the button's graphic in a calculated container field. If the calculation = true, the graphic will display. You can then overlay the graphic with a transparent box that is connected to your buttons script.

The downside to this is that you will need to have the same logic in the script to determine if the button is showing or not. If the user happens to click where the button is, when it's not showing, they will see the button "press" but your script will not run.

There is also another method that involves self-relations and portals. This method is much more complicated, but it actually "removes" the button object from the layout and cannot be clicked.

There is another way, maybe. You can put a button into a 1 line portal controlled by a relationship. If you can make the relationship work, i.e. indexed, the button will appear if the relationship is true, and disappear when it's false.