No you're not wasting electricity by plugging your phone charger in with the phone not charging on it.
The transformer for the cellphone charger is not using electricity until you plug the phone in to it so that it charges.
Even then the electricity used to charge your cellphone is so little that it won't even cost much more than $1.00 in an entire year of leaving your cellphone plugged in and charging all the time.
I always leave my cellphone plugged in when at home to keep it charged so I never worry about having a dead cellphone battery.
If a generator is spinning and the electricity is not being used then the electricity is just looped around in the generator.
However if the generator generates electricity and goes through the power grid and nobody uses it then the unused electricity goes back through the grid through the neutral wire and goes back into the power grid.
The unused electricity goes into the rotational energy of the generators at power plants across the grid.
A electricity generator at a power plant usually spins at a fixed rpm, which is also determined by the frequency of the grid and the design of the generator.
It's not really possible to waste electricity because the electricity that is being generated and not used just flows back into the generator through the neutral wire or goes back to someone else to use it.
So for example if the generator is being ran and generating electricity and nobody uses that electricity then that electricity is going to waste as it's using fuel to create that electricity.
In reality though you're wasting the fuel and not the actual electricity.
It's not really possible to waste electricity.
However when you do use more electricity it requires more electricity to be produced and to produce that electricity the generators have to work harder.
The more electricity you use the harder it is for the generators to spin to create the electricity.
For example if you have a portable electric gas powered generator and you start the engine and plug something into it.
During a light load such as with a fan, light bulb etc the generator will basically run at normal speed but the more load you put on the generator the faster the engine will run to create more power.
So you use more gasoline when you increase the electricity you use from your generator and when you turn off lights etc that are plugged into the generator the generators gasoline motor will slow down and use less fuel.
So you're not really wasting electricity but you're wasting fossil fuels or other fuels to spin that generator faster when you use more electricity.