Can I keep my charger plugged in?
Yes they will draw current and waste energy, but only very little.

I just measured two of my chargers I had lying around.

One used 0.104W and the other was around 0.09W with no load connected. Their power factor (\$\cos \phi\$) was pretty bad, around -0.33, so their VA was around 0.3VA. For these kind of powers we have no regulation in Germany which power factor has to be reached, but closer to 1 is always better.

I've used a Gossen Metrawatt Energy to measure that, just in case someone wonders.

So in a year one of them would use around 880Wh of energy for nothing. Sure if you put that in perspective to what I use over the whole year: around 1000kWh, it would only account to roughly 0,1%, so it doesn't matter much. It would cost me roughly 0.25€ per year.

I generally have a tendency to completely unpower any unused appliance, it saves energy from being wasted, but it also adds some safety. First the device may fail while I'm not around, which could be bad. And second if a thunderstorm comes around it may damage a lot more devices if I leave them all plugged in.