There is actually a perfectly straightforward word for this in English, and you’ve even included it in your question: shadowless.
The OED defines shadowless as –
- Casting no shadow.
- Of a lamp or its furniture: So constructed as to cast no shadow.
- Having no shadows on its surface; unsheltered from the sun. Of light, the sky, etc.: Unclouded.
The third of these meanings goes against the meaning you intend, of course; but the word is instantly recognisable and understandable, and though it’s rare, you should have no qualms about using it.
If you’re more interested in using a ‘Classical’ term, Ancient Greek has ἄσκιος, which has the exact same duality in meaning: either ‘unclouded/unshadowed’ or ‘casting no shadow’.
If there were a Latin word, it might be expected to be something like *inumbralis, but as far as I can tell, there isn’t. It would also be quite ambiguous in Latin, since inumbrō means ‘to overshadow’.