Fix handling of null labels (#4795)

Author: benmccann

src/core/core.scale.js

@@ -662,7 +662,7 @@ module.exports = function(Chart) {
 
 				// Since we always show the last tick,we need may need to hide the last shown one before
 				shouldSkip = (skipRatio > 1 && i % skipRatio > 0) || (i % skipRatio === 0 && i + skipRatio >= tickCount);
-				if (shouldSkip && i !== tickCount - 1 || helpers.isNullOrUndef(tick.label)) {
+				if (shouldSkip && i !== tickCount - 1) {
 					// leave tick in place but make sure it's not displayed (#4635)
 					delete tick.label;
 				}
@@ -712,7 +712,7 @@ module.exports = function(Chart) {
 
 			helpers.each(ticks, function(tick, index) {
 				// autoskipper skipped this tick (#4635)
-				if (tick.label === undefined) {
+				if (helpers.isNullOrUndef(tick.label)) {
 					return;
 				}