fix(builder): Image block placeholder no longer overridden by explicit src=""

BlocksPanel dropped a new Image block with an explicit `src=""` prop, which
overrides ImageBlock's `src = PLACEHOLDER_SRC` default parameter (defaults
only apply when a prop is undefined, not when it's an empty string). Craft
then persisted `src:''`, and the canvas rendered a broken-image icon instead
of the placeholder.

- ImageBlock render now falls back to PLACEHOLDER_SRC whenever src is falsy
  (belt-and-braces: also recovers any legacy saved src:'' state).
- BlocksPanel no longer passes src="" when dropping a new Image block, so
  the craft default applies.
- ImageStylePanel now restores the placeholder (instead of blanking to '')
  when the URL field is cleared.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/panels/right/styles/ImageStylePanel.tsx

@@ -11,6 +11,7 @@ import {
   useNodeProp,
 } from './shared';
 import { AssetPicker } from '../../../ui/AssetPicker';
+import { PLACEHOLDER_SRC } from '../../../components/media/ImageBlock';
 
 /* ---------- IMAGE (with upload/browse/drop) ---------- */
 export const ImageStylePanel: React.FC<StylePanelProps> = ({ selectedId, nodeProps }) => {
@@ -33,7 +34,7 @@ export const ImageStylePanel: React.FC<StylePanelProps> = ({ selectedId, nodePro
         <SectionLabel>Image Source</SectionLabel>
         <AssetPicker
           value={nodeProps.src || ''}
-          onChange={(url) => actions.setProp(selectedId, (props: any) => { props.src = url; })}
+          onChange={(url) => actions.setProp(selectedId, (props: any) => { props.src = url || PLACEHOLDER_SRC; })}
           variant="full"
         />
       </div>