The spec has in 19.496 style:list-style-name the text
<quote>
The list style specified by this attribute is applied to headings and paragraphs that are contained in a list, where the list does not specify a list style itself, and the list has no list style specification for any of its parents.
</quote>
But the spec does not say, what should happen, if the heading is not inside a list.

Look at
<quote>
An outline style is a list style that is applied to all headings within a text document where the heading's paragraph style does not define a list style to use itself.
</quote>
from 16.34 <text:outline-style>
Current implementations use this outline style for headings, which are not inside a list.

From these texts I guess, that it is possible that the headings paragraph style can define a list style to be used. I find no constrain, that the heading has to be inside a list.

The mail https://lists.oasis-open.org/archives/office/201707/msg00020.html has the files HeadingWithoutList.odt and StyleEmptyNoListInherit.odt attached, to test it.