Fix Markdown cards in Grid not taking full height

The implementation of show_empty property in #21379 introduced
regression which causes Markdown cards rendered within a Grid card to
not take full height of its space. This is because a visible card is now
forced to have "display: block", while without that it's rendered as
"display: inline".

As the CSSStyleDeclaration.style mandates string type, it's not possible
to delete or null the value. Setting it to an empty string seems to do
the trick as well and the linter is happy too.

Fixes #23119

src/panels/lovelace/cards/hui-markdown-card.ts

@@ -111,7 +111,7 @@ export class HuiMarkdownCard extends LitElement implements LovelaceCard {
       this._config.show_empty === false &&
       this._templateResult.result.length === 0;
     if (shouldBeHidden !== this.hidden) {
-      this.style.display = shouldBeHidden ? "none" : "block";
+      this.style.display = shouldBeHidden ? "none" : "";
       this.toggleAttribute("hidden", shouldBeHidden);
       fireEvent(this, "card-visibility-changed", { value: !shouldBeHidden });
     }