Verify that CssLinkResourceTransformer handles empty url() links

This commit introduces tests that verify that both
CssLinkResourceTransformer implementations properly handle empty url()
functions in CSS files.

See gh-22602

spring-webflux/src/test/java/org/springframework/web/reactive/resource/CssLinkResourceTransformerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,7 +37,9 @@ import static org.springframework.mock.http.server.reactive.test.MockServerHttpR
 
 /**
  * Unit tests for {@link CssLinkResourceTransformer}.
+ *
  * @author Rossen Stoyanchev
+ * @author Sam Brannen
  */
 public class CssLinkResourceTransformerTests {
 
@@ -159,6 +161,26 @@ public class CssLinkResourceTransformerTests {
 				.verify();
 	}
 
+	@Test // https://github.com/spring-projects/spring-framework/issues/22602
+	public void transformEmptyUrlFunction() throws Exception {
+		MockServerWebExchange exchange = MockServerWebExchange.from(get("/static/empty_url_function.css"));
+		Resource css = getResource("empty_url_function.css");
+		String expected =
+				".fooStyle {\n" +
+				"\tbackground: transparent url() no-repeat left top;\n" +
+				"}";
+
+		StepVerifier.create(this.transformerChain.transform(exchange, css)
+				.cast(TransformedResource.class))
+				.consumeNextWith(transformedResource -> {
+					String result = new String(transformedResource.getByteArray(), StandardCharsets.UTF_8);
+					result = StringUtils.deleteAny(result, "\r");
+					assertEquals(expected, result);
+				})
+				.expectComplete()
+				.verify();
+	}
+
 	private Resource getResource(String filePath) {
 		return new ClassPathResource("test/" + filePath, getClass());
 	}

spring-webflux/src/test/resources/org/springframework/web/reactive/resource/test/empty_url_function.css

@@ -0,0 +1,3 @@
+.fooStyle {
+	background: transparent url() no-repeat left top;
+}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,11 +34,11 @@ import org.springframework.web.servlet.resource.EncodedResourceResolver.EncodedR
 import static org.junit.Assert.*;
 
 /**
- * Unit tests for
- * {@link org.springframework.web.servlet.resource.CssLinkResourceTransformer}.
+ * Unit tests for {@link CssLinkResourceTransformer}.
  *
  * @author Rossen Stoyanchev
  * @author Brian Clozel
+ * @author Sam Brannen
  * @since 4.1
  */
 public class CssLinkResourceTransformerTests {
@@ -149,6 +149,21 @@ public class CssLinkResourceTransformerTests {
 		assertSame(gzipped, actual);
 	}
 
+	@Test // https://github.com/spring-projects/spring-framework/issues/22602
+	public void transformEmptyUrlFunction() throws Exception {
+		this.request = new MockHttpServletRequest("GET", "/static/empty_url_function.css");
+		Resource css = getResource("empty_url_function.css");
+		String expected =
+				".fooStyle {\n" +
+				"\tbackground: transparent url() no-repeat left top;\n" +
+				"}";
+
+		TransformedResource actual = (TransformedResource) this.transformerChain.transform(this.request, css);
+		String result = new String(actual.getByteArray(), StandardCharsets.UTF_8);
+		result = StringUtils.deleteAny(result, "\r");
+		assertEquals(expected, result);
+	}
+
 	private Resource getResource(String filePath) {
 		return new ClassPathResource("test/" + filePath, getClass());
 	}

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/empty_url_function.css

@@ -0,0 +1,3 @@
+.fooStyle {
+	background: transparent url() no-repeat left top;
+}