Fix for undefined source in Image layer

Prevent error if layer does not have a source.  Also clear any existing image if source is set to null or undefined by setSource.

src/ol/renderer/canvas/ImageLayer.js

@@ -55,6 +55,7 @@ class CanvasImageLayerRenderer extends CanvasLayerRenderer {
     }
 
     if (!hints[ViewHint.ANIMATING] && !hints[ViewHint.INTERACTING] && !isEmpty(renderedExtent)) {
+      if (imageSource) {
         let projection = viewState.projection;
         if (!ENABLE_RASTER_REPROJECTION) {
           const sourceProjection = imageSource.getProjection();
@@ -66,6 +67,9 @@ class CanvasImageLayerRenderer extends CanvasLayerRenderer {
         if (image && this.loadImage(image)) {
           this.image_ = image;
         }
+      } else {
+        this.image_ = null;
+      }
     }
 
     return !!this.image_;