Possible incorrect work of "metadata_value_of" with absent and unset metadata

The following index code shows problematic, it actually works so:

{% if document.metadata_value_of.some_data is none %}
some_data is required but not set
{% elif document.metadata_value_of.some_data == "" %}
some_data is not present
{% else %}
{{document.metadata_value_of.some_data}}
{% endif %}

Is it right to return empty string if metadata is not present? Is it somewhere documented?

Using Mayan-EDMS Version 3.2.7 from docker hub.