How can check if a field is empty in a node template?
Check on node.field_notes something like 



 {% set count_paragraph_element = node.field_notes | length %}
  {% if count_paragraph_element == 0 %}
      Empty
  {% else %}
      Not empty
  {% endif %}



Update
if you want to check if text plain long field has empty value try this.
I assume that your field machine name is field_text:



{% for item in node.field_notes %}
      {% if item.entity.field_text.value is empty %}
        text value is empty
      {% else %}
        text value not empty
      {% endif %}
{% endfor %}