Iterating over Var containing empty container throws "Out of range" exception

When iterating over `Poco::Dynamic::Var` that contains empty continer (e. g. `Poco::Dynamic::Vector`), iteration loop is entered and "Out of range" exception is thrown when dereferencing `VarIterator`.

```c++
Poco::Dynamic::Var v{Poco::Dynamic::Vector()};
int i = 0;
for (auto it = v.begin(); it != v.end(); ++it) {
    *it = ++i;
}
```