fix(color-contrast): support IE extension context (#2008)

msElementsFromPoint in IE extensions can return null instead of
an empty array.

lib/commons/dom/shadow-elements-from-point.js

@@ -14,7 +14,8 @@ dom.shadowElementsFromPoint = function(nodeX, nodeY, root = document, i = 0) {
 		throw new Error('Infinite loop detected');
 	}
 	return (
-		Array.from(root.elementsFromPoint(nodeX, nodeY))
+		// IE can return null when there are no elements
+		Array.from(root.elementsFromPoint(nodeX, nodeY) || [])
 			// As of Chrome 66, elementFromPoint will return elements from parent trees.
 			// We only want to touch each tree once, so we're filtering out nodes on other trees.
 			.filter(nodes => dom.getRootNode(nodes) === root)

test/commons/dom/shadow-elements-from-point.js

@@ -44,4 +44,17 @@ describe('dom.shadowElementsFromPoint', function() {
 			assert.notInclude(result2, shadowSpan);
 		}
 	);
+
+	it('does not throw when elementsFromPoints returns null', function() {
+		var mockDocument = {
+			elementsFromPoint: function() {
+				return null;
+			}
+		};
+		var out;
+		assert.doesNotThrow(function() {
+			out = axe.commons.dom.shadowElementsFromPoint(10, 10, mockDocument);
+		});
+		assert.deepEqual(out, []);
+	});
 });