Allow spell frame attributes without a unit

Currently, a button using ClassicAPI’s attribute system does nothing when `type="spell"` and no unit attribute is set:
```lua
local button = CreateFrame("Button")
button:SetAttribute("type", "spell")
button:SetAttribute("spell", "Lightwell")
button:Click()
```

Maybe we can fallback to `CastSpellByName` if unit is not present and use `C_Spell.CastAtUnit` otherwise?