fix(ui5-popover, ui5-dialog): do not render empty headers (#2770)

Fixes: #2694

Author: georgimkv

packages/main/src/Dialog.hbs

@@ -1,16 +1,18 @@
 {{>include "./Popup.hbs"}}
 
 {{#*inline "beforeContent"}}
-	<header
-		class="ui5-popup-header-root"
-		id="ui5-popup-header"
-		@mousedown="{{_onDragMouseDown}}">
-		{{#if header.length }}
-			<slot name="header"></slot>
-		{{else}}
-			<h2 class="ui5-popup-header-text">{{headerText}}</h2>
-		{{/if}}
-	</header>
+	{{#if _displayHeader }}
+		<header
+			class="ui5-popup-header-root"
+			id="ui5-popup-header"
+			@mousedown="{{_onDragMouseDown}}">
+			{{#if header.length }}
+				<slot name="header"></slot>
+			{{else}}
+				<h2 class="ui5-popup-header-text">{{headerText}}</h2>
+			{{/if}}
+		</header>
+	{{/if}}
 {{/inline}}
 
 {{#*inline "afterContent"}}

packages/main/src/Dialog.js

@@ -203,6 +203,10 @@ class Dialog extends Popup {
 		return "flex";
 	}
 
+	get _displayHeader() {
+		return this.header.length || this.headerText;
+	}
+
 	show() {
 		super.show();
 		this._center();

packages/main/src/Popover.js

@@ -659,7 +659,7 @@ class Popover extends Popup {
 	 * Hook for descendants to hide header.
 	 */
 	get _displayHeader() {
-		return true;
+		return this.header.length || this.headerText;
 	}
 
 	/**

packages/main/src/ResponsivePopover.js

@@ -163,7 +163,7 @@ class ResponsivePopover extends Popover {
 	}
 
 	get _displayHeader() {
-		return this._isPhone || !this.contentOnlyOnDesktop;
+		return (this._isPhone || !this.contentOnlyOnDesktop) && super._displayHeader;
 	}
 
 	get _displayFooter() {