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/components/media/ImageBlock.render.test.tsx

@@ -0,0 +1,62 @@
+import { describe, test, expect, vi, beforeEach } from 'vitest';
+import React from 'react';
+import { createRoot, Root } from 'react-dom/client';
+import { act } from 'react-dom/test-utils';
+
+/* ImageBlock only needs useNode from @craftjs/core. Mock it following the
+   DOM-harness pattern in src/components/basic/Footer.editguard.test.tsx (no
+   @testing-library/react in this repo) so we can render the real component
+   tree and inspect the emitted <img src> without a real <Editor>. */
+vi.mock('@craftjs/core', () => ({
+  useNode: (collect?: (node: any) => any) => {
+    const node = { events: { selected: false } };
+    return {
+      connectors: { connect: (el: any) => el, drag: (el: any) => el },
+      actions: { setProp: vi.fn() },
+      ...(collect ? collect(node) : {}),
+    };
+  },
+}));
+
+import { ImageBlock } from './ImageBlock';
+
+let container: HTMLDivElement;
+let root: Root;
+
+function render(ui: React.ReactElement) {
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  act(() => {
+    root = createRoot(container);
+    root.render(ui);
+  });
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+describe('ImageBlock render falls back to the placeholder for an explicit empty src (Bug 1)', () => {
+  test('src="" (explicit, overrides the default parameter) still renders a non-empty placeholder src', () => {
+    render(<ImageBlock src="" alt="Image" />);
+    const img = container.querySelector('img')!;
+    expect(img.getAttribute('src')).not.toBe('');
+    expect(img.getAttribute('src')).toMatch(/^data:image\/svg\+xml/);
+    container.remove();
+  });
+
+  test('src=undefined (default parameter path) still renders the placeholder (unchanged behavior)', () => {
+    render(<ImageBlock alt="Image" />);
+    const img = container.querySelector('img')!;
+    expect(img.getAttribute('src')).not.toBe('');
+    expect(img.getAttribute('src')).toMatch(/^data:image\/svg\+xml/);
+    container.remove();
+  });
+
+  test('a real src is rendered unchanged', () => {
+    render(<ImageBlock src="https://example.com/photo.jpg" alt="A photo" />);
+    const img = container.querySelector('img')!;
+    expect(img.getAttribute('src')).toBe('https://example.com/photo.jpg');
+    container.remove();
+  });
+});