The word 'antonym' needs defining before one can comment sensibly on this sort of question. There are different but related senses.
At http://grammar.about.com/od/ab/g/antonymsterms.htm , one class of antonyms is termed:
... Gradable Antonyms
Gradable antonyms include pairs like the following: beautiful - ugly
expensive - cheap fast - slow hot - cold increase - decrease long -
short love - hate rich - poor sweet - sour wide- narrow These pairs
are called gradable antonyms because they do not represent [refer to]
an either-or relation but rather a more-less relation. The words can
be viewed as terms at the end-points of a continuum or gradient.
Now while empty and full are essentially absolute adjectives, whereas say fast and slow are not, the former are certainly terms which refer to 'the end-points of a continuum or gradient'. They are gradable antonyms.