I only worked in NYC for this employer, so I'm unsure why box 18, 19 and 20 would be empty. The other two employers I worked for, also in NYC, have information in those three fields. Is this an error on the employers behalf?
If boxes 18,19 & 20 are blank on the Form W-2, that employer did not withhold NYC income tax from your wages.
This could be the case for a number of reasons (e.g. your income from that employer was too low to require them to withhold NYC tax, or they weren't set up to withhold the NYC tax).
As a resident of NYC, you would still be responsible for NYC tax in the income from that employer.
You may want to contact that employer's the payroll department (or preparer) and ask why NYC taxes were not withheld. If you are still an employee of that company, you should ask them to start withholding them from your wages.