Fix min and max option checks in linear scales (chartjs#5209)

When 0, the min and max options was considered not being set, instead we should check for null or undefined.

src/scales/scale.linearbase.js

@@ -36,7 +36,7 @@ function generateTicks(generationOptions, dataRange) {
 	var niceMax = Math.ceil(dataRange.max / spacing) * spacing;
 
 	// If min, max and stepSize is set and they make an evenly spaced scale use it.
-	if (generationOptions.min && generationOptions.max && generationOptions.stepSize) {
+	if (!helpers.isNullOrUndef(generationOptions.min) && !helpers.isNullOrUndef(generationOptions.max) && generationOptions.stepSize) {
 		// If very close to our whole number, use it.
 		if (helpers.almostWhole((generationOptions.max - generationOptions.min) / generationOptions.stepSize, spacing / 1000)) {
 			niceMin = generationOptions.min;

test/specs/scale.linear.tests.js

@@ -965,4 +965,60 @@ describe('Linear Scale', function() {
 		expect(chart.scales['x-axis-0'].min).toEqual(20);
 		expect(chart.scales['x-axis-0'].max).toEqual(21);
 	});
+
+	it('min settings should be used if set to zero', function() {
+		var barData = {
+			labels: ['S1', 'S2', 'S3'],
+			datasets: [{
+				label: 'dataset 1',
+				backgroundColor: '#382765',
+				data: [2500, 2000, 1500]
+			}]
+		};
+
+		var chart = window.acquireChart({
+			type: 'horizontalBar',
+			data: barData,
+			options: {
+				scales: {
+					xAxes: [{
+						ticks: {
+							min: 0,
+							max: 3000
+						}
+					}]
+				}
+			}
+		});
+
+		expect(chart.scales['x-axis-0'].min).toEqual(0);
+	});
+
+	it('max settings should be used if set to zero', function() {
+		var barData = {
+			labels: ['S1', 'S2', 'S3'],
+			datasets: [{
+				label: 'dataset 1',
+				backgroundColor: '#382765',
+				data: [-2500, -2000, -1500]
+			}]
+		};
+
+		var chart = window.acquireChart({
+			type: 'horizontalBar',
+			data: barData,
+			options: {
+				scales: {
+					xAxes: [{
+						ticks: {
+							min: -3000,
+							max: 0
+						}
+					}]
+				}
+			}
+		});
+
+		expect(chart.scales['x-axis-0'].max).toEqual(0);
+	});
 });