Polish WebJarsResourceResolver

Fix a potential NPE when trying to resolve non-existing webjars
resources in the resolver chain.

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/WebJarsResourceResolver.java

@@ -75,7 +75,9 @@ public class WebJarsResourceResolver extends AbstractResourceResolver {
 		String path = chain.resolveUrlPath(resourceUrlPath, locations);
 		if (path == null) {
 			String webJarResourcePath = findWebJarResourcePath(resourceUrlPath);
-			return chain.resolveUrlPath(webJarResourcePath, locations);
+			if(webJarResourcePath != null) {
+				return chain.resolveUrlPath(webJarResourcePath, locations);
+			}
 		}
 		return path;
 	}

spring-webmvc/src/test/java/org/springframework/web/servlet/resource/WebJarsResourceResolverTests.java

@@ -107,6 +107,7 @@ public class WebJarsResourceResolverTests {
 
 		assertNull(actual);
 		verify(this.chain, times(1)).resolveUrlPath(file, this.locations);
+		verify(this.chain, never()).resolveUrlPath(null, this.locations);
 	}
 
 	@Test