Fix error with display of Heading block style variations in style book. (#72551)

Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org> Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

Author: tellthemachines

packages/edit-site/src/components/style-book/index.js

@@ -224,19 +224,29 @@ function applyBlockVariationsToExamples( examples, variation ) {
 if ( ! variation ) {
 return examples;
 }
-
-return examples.map( ( example ) => ( {
-...example,
-variation,
-blocks: {
-...example.blocks,
-attributes: {
-...example.blocks.attributes,
-style: undefined,
-className: getVariationClassName( variation ),
-},
-},
-} ) );
+return examples.map( ( example ) => {
+return {
+...example,
+variation,
+blocks: Array.isArray( example.blocks )
+? example.blocks.map( ( block ) => ( {
+...block,
+attributes: {
+...block.attributes,
+style: undefined,
+className: getVariationClassName( variation ),
+},
+ } ) )
+: {
+...example.blocks,
+attributes: {
+...example.blocks.attributes,
+style: undefined,
+className: getVariationClassName( variation ),
+},
+ },
+};
+} );
 }
 
 function StyleBook( {