Am I getting you right: You want to route the message one way if there is at least one empty Field1in the message and the other way if there is no empty Field1 present?
Maybe you could try:
Left: boolean(/Message/Body[Field1 = '']/
Operator: =
Right: true
Left: boolean(/Message/Body[Field1 = '']/
Operator: !=
Right: false
boolean() should return true if the node-set provided to the function contains at least one node.
I think you should use single quotes for literals.
I'm not sure about the slash at the end of the left operator.