Drop NULL children in layout_column_wrap() (#560)

Author: gadenbuie

R/layout.R

@@ -77,6 +77,7 @@ layout_column_wrap <- function(
   #      to fill the grid row.
   #   2. Allow for heights_equal="-cell", which useful for allowing contents to
   #      shrink but not grow (i.e., default flex behavior).
+  children <- dropNulls(children)
   children <- lapply(children, function(x) {
     bindFillRole(
       container = TRUE,