VsiteHandler::constructPositions skips entries without parameters

The vsitehandler skips constructing positions when there are no parameters. But it really should not be called before the parameters are assigned. At present the warning only shows up in the debug files:
```
if (params.empty())
{
if (debug)
{
fprintf(debug, "WARNING: No parameters to generate virtual sites in topology (yet).\n");
}
continue;
}
```