Is it better to unplug my MacBook Air while I am away?

Apple has the advice below for such a situation.

Store it half-charged when you store it long term.

If you want to store your device long term, two key factors will affect the overall health of your battery: the environmental temperature and the percentage of charge on the battery when it’s powered down for storage. Therefore, we recommend the following:

• Do not fully charge or fully discharge your device’s battery — charge it to around 50%. If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge. Conversely, if you store it fully charged for an extended period of time, the battery may lose some capacity, leading to shorter battery life.
• Power down the device to avoid additional battery use.
• Place your device in a cool, moisture-free environment that’s less than 90° F (32° C).
• If you plan to store your device for longer than six months, charge it to 50% every six months.

Depending on how long you store your device, it may be in a low-battery state when you remove it from long-term storage. After it’s removed from storage, it may require 20 minutes of charging with the original adapter before you can use it.

The only thing I would add to @stedman1's excellent reply would be the following:

Disconnect all external devices prior to powering down the laptop. This includes the power adapter. Sometimes when disconnecting an external device it will end up waking or powering on the laptop.

Also, prior to powering down the laptop, press the Caps Lock key so the Caps Lock LED is lit. When the system finally powers down fully the Caps Lock LED will go out.

Make sure not to touch any keys or even the Trackpad as that can trigger the laptop to power back on again.

Once you close the Clamshell Lid, do not open it again or you will power on the laptop.