Generally taken that you can't, not a problem for Artificers as all their spells have M
You're right that there's no provision for using a spellcasting focus on spells without a material component. As the rules are generally taken as defined positively (ie. describing what you can do, as opposed to what you can't do), this is often taken to mean you can't use a spellcasting focus if the spell doesn't have a material component. This leads to some amount of dancing around getting a hand free, but (in my experience and as a DM) a lot of tables don't care too much about components unless it's actually interesting in the situation (silence, underwater, etc.).
When it comes to specific magic items, which is where getting to use focus on non-M spells starts really being a boon, it is worth noting that a number of magic items, eg. the Arcane Grimoire from Tasha's, boosts the DC from it being held, independently of whether you use it as a focus.
While you are holding this leather-bound book, you can use it as a spellcasting focus for your wizard spells, and you gain a bonus to spell attack rolls and to the saving throw DCs of your wizard spells.
And specifically for Artificers, this is simplified because of the specifics of their spellcasting feature. Artificers cast all their spells through their spellcasting focus, even if their normally don't have a material component (note that the clarifying parenthesis was added with the Tasha's printing):
You must have a spellcasting focus—specifically thieves’ tools or some kind of artisan’s tool—in hand when you cast any spell with this Spellcasting feature (meaning the spell has an “M” component when you cast it).
Feature's such as Arcane Firearm expand what you can use as a spellcasting focus (creating an exception to the general rule), you still need to use a focus, meaning it has an M component. (As a wand is not a tool, you presumably don't need to have the non-existent wand proficiency, as the feature wouldn't work if you had.)