Bug fix: handling of NAs in tranasition_filter

[rsaporta]

PROBLEM:

When there are NAs in the columns used for filtering in transition_filter, possible error:

Error in `$<-.data.frame`(`*tmp*`, "colour", value = "grey") :
  replacement has 1 row, data has 0

Issue tracks down to an if() clause in the assign_filters() function.

SOLUTION:

add na.rm = TRUE to the if clause argument

[thomasp85]

Thank you 🙏