Changing the conditional expression to check for undefined or null headers

Co-authored-by: Mustafa Zengin <muzengin@microsoft.com>

Author: nikithauc

src/GraphRequest.ts

@@ -310,7 +310,7 @@ export class GraphRequest {
 	 * @returns nothing
 	 */
 	private setHeaderContentType(): void {
-		if (this._headers === undefined || this._headers === null) {
+		if (!this._headers) {
 			this.header("Content-Type", "application/json");
 		}
 		let isContentTypePresent = false;