vm/tests: do not load empty script

In case we load a zero-length script, VM should end in HALT state.
https://github.com/neo-project/neo-vm/blob/master/tests/neo-vm.Tests/Tests/Others/OtherCases.json#L22

pkg/vm/vm_test.go

@@ -2455,7 +2455,9 @@ func makeProgram(opcodes ...opcode.Opcode) []byte {
 
 func load(prog []byte) *VM {
 	vm := New()
+	if len(prog) != 0 {
 		vm.LoadScript(prog)
+	}
 	return vm
 }