fix: fix property event handlers when setting null value to property.

[andycall]

修复执行 element.onclick = null 而抛出 JS 异常的问题，并且添加对应的实现。

[openkraken-bot]

• Total time cost(without paint and layout): 666.84ms ↓ (3%)
• widget_creation_cost: 39.11ms ↓ (2%)
  • controller_properties_init_cost 0.00ms(0%)
  • view_controller_properties_init_cost 0.00ms(0%)
  • element_manager_init_cost 0.00ms(0%)
  • element_manager_property_init 0.00ms(0%)
  • root_element_property_init_cost 0.00ms(0%)
  • root_element_init_cost 0.00ms(0%)
  • create_viewport 0.00ms(0%)
  • bridge_init_cost 0.00ms(0%)
  • bridge_register_dart_method_cost 0.00ms(0%)
  • js_context_init_cost 0.00ms(0%)
  • native_method_init_cost 0.00ms(0%)
  • polyfill_init_cost 0.00ms(0%)
• First Bundle Load: 626.735ms ↓ (3%)
  • js_bundle_load_cost 0.00ms(0%)
  • js_bundle_eval_cost 0.00ms(0%)
  • js_parse_time_cost 0.00ms(0%)
  • flush_ui_command_cost 20.00ms(0%) avg: 0.76ms↑ (38%)
  • create_element_cost 195.00ms(0%) avg: 0.01ms(0%)
  • js_host_class_get_property_cost 5734.00ms(0%) avg: 0.01ms(0%)
  • js_host_class_set_property_cost 2735.00ms(0%) avg: 0.02ms(0%)
  • js_host_class_init_cost 0.00ms(0%)
  • js_native_function_call_cost 1843.00ms(0%) avg: 0.03ms(0%)
  • create_text_node_cost 47.00ms(0%) avg: 0.01ms(0%)
  • create_comment_cost 116.00ms(0%)
  • dispose_event_target_cost 0.00ms(0%)
  • add_event_cost 341.00ms(0%) avg: 0.04ms↑ (100%)
  • insert_adjacent_node_cost 356.00ms(0%) avg: 0.08ms(0%)
  • remove_node_cost 5.00ms(0%)
  • set_style_cost 2054.00ms(0%) avg: 0.01ms(0%)
  • dom_force_layout_cost 5.00ms(0%) avg: 13.50ms↓ (1%)
  • dom_flush_ui_command_cost 143.00ms(0%) avg: 0.54ms↓ (1%)
  • set_properties_cost 860.00ms(0%) avg: 0.01ms↓ (49%)
  • remove_properties_cost 0.00ms(0%)
• Rendering: 66.62ms ↓ (10%)
  • flex_layout_cost 253.50ms↓ (0%) avg: 0.11ms↓ (4%)
  • flow_layout_cost 93.00ms↓ (4%) avg: 0.07ms↓ (14%)
  • intrinsic_layout_cost 124.00ms(0%) avg: 0.01ms(0%)
  • silver_layout_cost 0.00ms(0%)
  • paint_cost 1579.00ms↓ (5%) avg: 0.02ms(0%)