ReflectionParameter::getTypehintText() returns empty string for nullable types

### HHVM Version
```
HipHop VM 3.15.2 (rel)
Compiler: tags/HHVM-3.15.2-0-g83ac3e5e3f5657be0cf4c55884044f86a7818b90
Repo schema: 608339137764e8365964a1adaa7a27d125b6076f
```
### Standalone code, or other way to reproduce the problem
```php
getTypehintText(), $y->getTypehintText());
```
### Expected result
```
string(6) "HH\int"
string(6) "HH\int"
```
### Actual result
```
string(6) "HH\int"
string(0) ""
```