Fix NPE when normalizing client tag URIs

See gh-11798

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/web/client/MetricsClientHttpRequestInterceptor.java

@@ -105,7 +105,7 @@ class MetricsClientHttpRequestInterceptor implements ClientHttpRequestIntercepto
 	}
 
 	private String ensureLeadingSlash(String url) {
-		return (url.startsWith("/") ? url : "/" + url);
+		return (url == null || url.startsWith("/") ? url : "/" + url);
 	}
 
 }