Doesn't try to combine labels if labels are NULL (#4115)

* Doesn't try to combine labels if labels are NULL

* Changes & to &&

Author: eliocamp

R/coord-polar.r

@@ -281,7 +281,7 @@ CoordPolar <- ggproto("CoordPolar", Coord,
     # Combine the two ends of the scale if they are close
     theta <- theta[!is.na(theta)]
     ends_apart <- (theta[length(theta)] - theta[1]) %% (2*pi)
-    if (length(theta) > 0 && ends_apart < 0.05) {
+    if (length(theta) > 0 && ends_apart < 0.05 && !is.null(labels)) {
       n <- length(labels)
       if (is.expression(labels)) {
         combined <- substitute(paste(a, "/", b),