Empty graph fails to plot

```r
library(ergm)
library(netplot)

n <- 50
n_egos <- 10
net <- as.network(matrix(0, ncol = n, nrow = n), directed = TRUE)
nplot(net)
# Error in quantile.default(netenv$vertex.size, 1 - netenv$vertex.label.show[1]) : 
# missing values and NaN's not allowed if 'na.rm' is FALSE
```