Check constructor is defined when constructor set null

When constructor is null, type-name throws exception like `TypeError: Cannot read property 'name' of null`.

```
TypeError: Cannot read property 'name' of null
      at funcName (/Users/furukawa.yosuke/Program/type-name/index.js:15:13)
      at ctorName (/Users/furukawa.yosuke/Program/type-name/index.js:21:16)
      at typeName (/Users/furukawa.yosuke/Program/type-name/index.js:33:16)
```

traceur-compiler sets null constructor in some classes...