Fixes compiler error

[bhelx]

I'm getting this error using asc 0.21.3:

INFO AS210: Expression is never 'null'.
    :
 82 │ this.writeString(str!);
    │                  ~~~
    └─ in ~lib/assemblyscript-json/assembly/encoder.ts(82,24)

I believe it's because there is already a null check on the line above. This change fixed the problem for me.