descriptor: refuse to parse unspendable miniscript descriptors

It's possible for some unsatisfiable miniscripts to be considered sane.
Make sure we refuse to import those, as they would be unspendable.

src/script/miniscript.h

@@ -1161,6 +1161,9 @@ public:
         return true;
     }
 
+    //! Whether no satisfaction exists for this node.
+    bool IsNotSatisfiable() const { return !GetStackSize(); }
+
     //! Return the expression type.
     Type GetType() const { return typ; }