Null check on isVoidVoid()

In case the current `type` was null (is sometimes the case 
when return type hints are not provided in the code but are 
left for type inference to figure out)

Without this check I was receiving this error:

```
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:865: characters 10-14 : Invalid field access : index
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:1000: characters 11-28 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:848: characters 13-42 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:517: characters 5-28 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:491: characters 3-29 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:329: characters 15-29 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:243: characters 19-41 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:99: characters 4-25 : Called from
<builtin>:1: character 0 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/macro/MockMaker.hx:27: lines 27-1064 : Called from
/usr/lib/haxe/lib/mockatoo/3,1,1/mockatoo/Mockatoo.hx:25: characters 13-58 : Called from
src/ufront/test/TestUtils.hx:58: characters 13-31 : Called from
```

It might also be preferable to have a requirement that mocked classes use complete type signiatures on all methods and variables.  If this is what you'd prefer let me know and I'll create a pull request for that instead.