skip data concatenation for z instruction in collapseRepeated

Example path part:

```
-32.5 -82zt0 0zM371
```

It will be stripped to:

```
-32.5 -82zzM371
```

So it will be crashed (with no output) in convertPathData.js::collapseRepeated:

```js
prev.data = prev.data.concat(item.data);
```

Full example to reproduce:

```xml
<svg xmlns="http://www.w3.org/2000/svg">
    <path d="M213 543q0 -41 20 -66.5q20 -25.5 50 -45.5l49 228q-54 -4 -86.5 -34q-32.5 -30 -32.5 -82zt0 0zM371 48z" />
</svg>
```