Handle qp clear input correctly (#3258)

* handle clearing xml in qp view

* formatting

src/sql/parts/queryPlan/queryPlan.ts

@@ -103,6 +103,7 @@ export class QueryPlan {
 	public set xml(xml: string) {
 		this._xml = xml;
 		new Builder(this.container).empty();
+		if (this.xml) {
 			QP.showPlan(this.container, this._xml, {
 				jsTooltips: false
 			});
@@ -111,6 +112,7 @@ export class QueryPlan {
 				toolTip.classList.add('monaco-editor-hover');
 			});
 		}
+	}
 
 	public get xml(): string {
 		return this._xml;