100ml Shortfill E-liquids are typically supplied in a 120ml bottle containing 100ml of nicotine free e-liquid. This leaves you 20ml of space to add two, 10ml nicotine shots. Adding two nic shot will produce a 3mg mix.
Absolutely, it’s perfectly fine to vape 100ml Shortfills without adding any nicotine, this is also known as 0mg vaping. The flavour may be more intense due to the shortfill not being diluted with nicotine shots, however extra flavour can certainly be regarded as a positive, not a negative.