Add required default markdown visState (#38390)

The markdownVis expression has the markdown string property configured as required.The current implementation of the markdown vis doesn't have a default value (actually it's undefined) and this cause the error to be thrown when creating a new markdown vis.

fix #38127

src/legacy/core_plugins/markdown_vis/public/markdown_vis.js

@@ -43,7 +43,8 @@ function MarkdownVisProvider() {
       component: MarkdownVisWrapper,
       defaults: {
         fontSize: 12,
-        openLinksInNewTab: false
+        openLinksInNewTab: false,
+        markdown: '',
       }
     },
     editorConfig: {

src/legacy/ui/public/visualize/loader/pipeline_helpers/build_pipeline.test.js

@@ -92,6 +92,12 @@ describe('visualize loader pipeline helpers: build pipeline', () => {
       expect(actual).toMatchSnapshot();
     });
 
+    it('handles undefined markdown function', () => {
+      const params = { fontSize: 12, openLinksInNewTab: true, foo: 'bar' };
+      const actual = buildPipelineVisFunction.markdown({ params });
+      expect(actual).toMatchSnapshot();
+    });
+
     describe('handles table function', () => {
       it('without splits or buckets', () => {
         const params = { foo: 'bar' };

src/legacy/ui/public/visualize/loader/pipeline_helpers/build_pipeline.ts

@@ -226,7 +226,10 @@ export const buildPipelineVisFunction: BuildPipelineVisFunction = {
   },
   markdown: visState => {
     const { markdown, fontSize, openLinksInNewTab } = visState.params;
-    const escapedMarkdown = escapeString(markdown);
+    let escapedMarkdown = '';
+    if (typeof markdown === 'string' || markdown instanceof String) {
+      escapedMarkdown = escapeString(markdown.toString());
+    }
     let expr = `markdownvis '${escapedMarkdown}' `;
     if (fontSize) {
       expr += ` fontSize=${fontSize} `;