Is electrical energy wasted when a device is fully charged?
Yes, some energy will still be consumed.

Assuming the laptop is switched off (not in sleep mode) and the battery is fully charged, it will consume very little energy (a few Watt perhaps).

When in sleep mode, the laptop keeps the RAM powered so it will consume somewhat more power.

But do not neglect the power adapter (many call this the charger but it is not, the charging circuit is inside the laptop). If the power adapter is of good quality it will also consume very little energy.

You can easily check what is taking significant power as this power is converted into heat. So an inefficient power adapter will get warm or even hot when you charge the laptop. When the laptop is off the power adapter should not feel warm (allow it some time to cool down after charging though).