code: def test_empty_locations(self):
      symbols = symbol_layer([])
      assert symbols.markers == []
  
imports: import unittest
  import pytest
  import numpy
  import traitlets
  from ..marker import MarkerOptions, Marker, Markers, Symbol, marker_layer, symbol_layer
label_desc: Use image node_id 4 for calling the SymbolLayer obj's underlying member method code with example usage: obj.test_empty_locations() without return types