generateDesignOfDefaults fails with vector params of undefined length.

Example:
```R
> x = makeParamSet(makeIntegerVectorLearnerParam("test", default = c(0, 0, 1),
+   len = NA, lower = 0, upper = 1))
> generateDesignOfDefaults(x)
Error in rep(types, getParamLengths(par.set)) : invalid 'times' argument
```
This should probaby also work when `default` has length 0.

(FYI I don't need this personally, but thought it is good to know.)