Fix error in safeLabel for formula label with empty data

[mz8i]

When adding an empty simple features dataframe to a map and specifying a formula-based label, methods like addPolygons error, because safeLabel fails on empty inputs with

Error in sum(sapply(label, function(x) { : 
  invalid 'type' (list) of argument

(tries to sum an empty list)

Example of failing safeLabel call

safeLabel(~X, data.frame(X=1:10) %>% head(0))

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
0 out of 2 committers have signed the CLA.

:x: Maciej Ziarkowski
:x: mz8i

---

Maciej Ziarkowski seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}