initialize repeated enums with []

This solves a traceback for parsing relation MemberTypes of the OSM PBF schema:
```
undefined:70
        obj.types.push(enc[6].decode(buf, offset))
                  ^
TypeError: Object 0 has no method 'push'
    at Object.decode (eval at <anonymous> (osm-pbf-parser/node_modules/protocol-buffers/node_modules/generate-function/index.js:1:0), <anonymous>:70:19)
    at Object.decode (eval at <anonymous> (osm-pbf-parser/node_modules/protocol-buffers/node_modules/generate-function/index.js:1:0), <anonymous>:37:33)
    at Object.decode (eval at <anonymous> (osm-pbf-parser/node_modules/protocol-buffers/node_modules/generate-function/index.js:1:0), <anonymous>:28:38)
    at osm-pbf-parser/index.js:81:56
```

I must admit that I don't really know what I'm doing here. This code is way too mad sciency.