Render tables with no columns correctly

[bantic]

Fixes #735.

When a table has no columns, the root ColumnTreeNode's isLeaf property was mistakenly evaluated as true because it (the columnTreeNode) itself has no subcolumns. As a result, when ensureWidthConstraint is called and the ColumnTree iterates the columns to tally their width, the meta for the root node is incorrectly retrieved:

ember-table/addon/-private/column-tree.js

Lines 327 to 337 in 024b2af

	if (get(this, 'isLeaf')) {
	let column = get(this, 'column');
	let columnWidth = get(column, 'width');
	if (typeof columnWidth === 'number') {
	return columnWidth;
	} else {
	let meta = get(this, 'tree.columnMetaCache').get(column);
	return get(meta, '_width');
	}
	}

There is no meta for the root ColumnTreeNode, though, so the attempt to get(meta, '_width') throws: Assertion Failed: Cannot call get with '_width' on an undefined object.

The root column tree node should not ever be treated as a leaf, so this PR ensures that it won't be.