The spec has in 19.496 style:list-style-name the text
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.
But the spec does not say, what should happen, if the heading is not inside a list.
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.
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.