PathResourceResolver should not resolve root path

When resolving resources, the PathResourceResolver creates a Resource
instance and checks whether this resource `exists()` and `isReadable()`.
While that last call returns false for folders on the file system, both
calls return true for folders located inside JARs.

If a JAR location is configured as a resource location, then
PathResourceResolver can resolve folders in JARs as valid locations and
candidates for paths resolution.

Prior to this change, the PathResourceResolver would resolve "" as a
valid resource path (here, the "/META-INF/resources/webjars" if
configured, for example) and return a "" path for this resource,
effectively turning all "/" URLs into empty ones "".

This commit fixes the resolveUrlPathInternal implementation by not
allowing empty paths as valid resource paths.

Issue: SPR-13241

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

@@ -124,4 +124,15 @@ public class PathResourceResolverTests {
 		assertTrue(this.resolver.checkResource(resource, resource));
 	}
 
+	// SPR-13241
+	@Test
+	public void resolvePathRootResource() throws Exception {
+		Resource webjarsLocation = new ClassPathResource("/META-INF/resources/webjars/", PathResourceResolver.class);
+		Resource actual = this.resolver.resolveResource(null, "", Arrays.asList(webjarsLocation), null);
+		String path = this.resolver.resolveUrlPathInternal("", Arrays.asList(webjarsLocation), null);
+
+		assertNotNull(actual);
+		assertTrue(actual.exists() && actual.isReadable());
+		assertNull(path);
+	}
 }