Missing return in lamdas confuses JDT

This is similar to a problem described already here:
- https://github.com/eclipse-jdt/eclipse.jdt.ui/issues/759

Assume the following code:

```
CompletableFuture.supplyAsync(()->{

});
```

This results in these quick fixes:


of course neither of those produce any useful results, the first results in

```
CompletableFuture.supplyAsync((Supplier)()->{

});
```

the second (now with the same error):
```
CompletableFuture.thenApplyAsync(()->{

});
```

if one instead performs the refactoring "Convert to anonymous class creation",


the problem becomes a bit more clear:

```
CompletableFuture.supplyAsync(new Supplier() {
@Override
public U get() {

}
});
```

even though `U` is not really a valid choice, so we replace `U` with `Object`:

```
CompletableFuture.supplyAsync(new Supplier<>() {

@Override
public Object get() {

}
});
```
Finally we got what we would like have seen in the first place, **adding the missing return statement!**


Now one can choose to "convert to lamda expression" refactoring to get to a working state again...