Fix keyboard navigation when the contentNode is focused

In a grid with few enough rows that there is empty space below them the user
can click below the rows causing the grid's contentNode to receive focus.
If the arrow keys are then pressed to attempt cell/row navigation an error
is thrown. This change checks for and ignores selection of the contentNode.

It specifically checks for contentNode instead of the more resilient/generic
"not a row or cell" because that condition is more difficult to determine.
grid.row() returns undefined for grid.contentNode, but also for any node
within the header. Header navigation logic depends on this.

Fixes #1297

Keyboard.js

@@ -441,6 +441,12 @@ var moveFocusVertical = Keyboard.moveFocusVertical = function(event, steps){
 	// if there is no _focusNode (for example, when the grid doesn't have data) don't try to find the next focus row/cell
 	if (!this._focusOnNode) { return; }
 
+	// don't attempt navigation if a cell or row is not currently focused
+	// this can happen if empty space within the grid has been clicked
+	if (event.target === this.contentNode) {
+		return;
+	}
+
 	var cellNavigation = this.cellNavigation,
 		target = this[cellNavigation ? "cell" : "row"](event),
 		columnId = cellNavigation && target.column.id,
@@ -471,6 +477,13 @@ var moveFocusPageDown = Keyboard.moveFocusPageDown = function(event){
 
 var moveFocusHorizontal = Keyboard.moveFocusHorizontal = function(event, steps){
 	if(!this.cellNavigation){ return; }
+
+	// don't attempt navigation if a cell or row is not currently focused
+	// this can happen if empty space within the grid has been clicked
+	if (event.target === this.contentNode) {
+		return;
+	}
+
 	var isHeader = !this.row(event), // header reports row as undefined
 		currentNode = this["_focused" + (isHeader ? "Header" : "") + "Node"];