Bug 53881 [Colors] Color/Gradient registry should return default values.
There should be default values for when a user does .get() on a bad key.
bundles/org.eclipse.jface/src/org/eclipse/jface/resource/GradientRegistry.java
/**
	 * This registries <code>Display</code>. All gradients will be allocated using 
	 * it.
	 */
	protected Display display;
	/**
	 * Collection of <code>Gradient</code> that are now stale to be disposed when 
	 * it is safe to do so (i.e. on shutdown).
	 */
	private List staleGradients = new ArrayList();
	/**
	 * Table of known gradients, keyed by symbolic gradient name (key type: <code>String</code>,
	 * value type: <code>Gradient</code>.
	 */
	private Map stringToGradient = new HashMap(7);
	/**
	 * Table of known gradient data, keyed by symbolic gradient name (key type:
	 * <code>String</code>, value type: <code>GradientData</code>).
	 */
	private Map stringToGradientData = new HashMap(7);
	
	/**
	 * Runnable that cleans up the manager on disposal of the display.
	 */