A couple of observations around these lines…

if fizzBuzz == "" {
    fizzBuzz += "\(i)"
}

• This seems like an unjustified use of string interpolation. String(i) would be more direct.
• Why bother concatenating to an empty string?

With those two changes…

if fizzBuzz == "" {
    fizzBuzz = String(i)
}

A couple of observations around these lines…

if fizzBuzz == "" {
    fizzBuzz += "\(i)"
}

• This seems like an unjustified use of string interpolation. String(i) would be more direct.
• Why bother concatenating to an empty string?

With those two changes…

if fizzBuzz == "" {
    fizzBuzz = String(i)
}