Prevent EmptyStackException in case no stack of prompts is empty

Author: cdupuis

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/shell/ShellPrompts.java

@@ -43,7 +43,9 @@ public void pushPrompt(String prompt) {
 	 * @see #pushPrompt(String)
 	 */
 	public void popPrompt() {
-		this.prompts.pop();
+		if (!this.prompts.isEmpty()) {
+			this.prompts.pop();
+		}
 	}
 
 	/**