If there was a year 0, we would still be in the 21st century. It would just have started 1 year earlier, in 2000 instead of 2001:

0 – 99: 1st century

100 – 199: 2nd century

...

1900 – 1999: 20th century

2000 – 2999: 21st century

Although I think centuries are usually treated as starting in years ending in 0 anyway, in casual settings. Most people were certainly happy to celebrate "the new millennium" at the start of the year 2000.