dnd 5e – Can a spellcasting focus be used in the casting of a spell with no Material components

It doesn’t matter; flavour however you like

Not all spells need objects. If a spell does not require a material component, then no spell casting focus is required.

From the PHB on casting spells and components

Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5, “Equipment”) in place of the components specified for a spell.

For your specific example, it’s not that you “cannot use” the feature, it’s that it’s not relevant to the spell you are casting. For instance, the spell Thunderclap is Somatic only; no verbal, no material. Just wiggle your fingers and it happens.