Fix RootUriTemplateHandler to apply rootUri for empty URI templates

module/spring-boot-restclient/src/main/java/org/springframework/boot/restclient/RootUriTemplateHandler.java

@@ -30,6 +30,7 @@
  *
  * @author Phillip Webb
  * @author Scott Frederick
+ * @author Hyunwoo Gu
  * @since 4.0.0
  */
 public class RootUriTemplateHandler implements UriTemplateHandler {
@@ -64,7 +65,9 @@ public URI expand(String uriTemplate, @Nullable Object... uriVariables) {
 	String apply(String uriTemplate) {
 		String rootUri = getRootUri();
 		if (rootUri != null && StringUtils.startsWithIgnoreCase(uriTemplate, "/")) {
-			return getRootUri() + uriTemplate;
+			return rootUri + uriTemplate;
+		} else if (rootUri != null && uriTemplate.isBlank()) {
+			return rootUri;
 		}
 		return uriTemplate;
 	}

module/spring-boot-restclient/src/test/java/org/springframework/boot/restclient/RootUriBuilderFactoryTests.java

@@ -19,6 +19,7 @@
 import java.net.URI;
 import java.net.URISyntaxException;
 
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.web.util.UriBuilder;
@@ -32,15 +33,32 @@
  * Tests for {@link RootUriBuilderFactory}.
  *
  * @author Scott Frederick
+ * @author Hyunwoo Gu
  */
 class RootUriBuilderFactoryTests {
 
+	private String rootUri;
+
+	private UriBuilderFactory builderFactory;
+
+	@BeforeEach
+	void setUp() {
+		this.rootUri = "https://example.com";
+		this.builderFactory = new RootUriBuilderFactory(this.rootUri, mock(UriTemplateHandler.class));
+	}
+
 	@Test
 	void uriStringPrefixesRoot() throws URISyntaxException {
-		UriBuilderFactory builderFactory = new RootUriBuilderFactory("https://example.com",
-				mock(UriTemplateHandler.class));
-		UriBuilder builder = builderFactory.uriString("/hello");
+		UriBuilder builder = this.builderFactory.uriString("/hello");
 		assertThat(builder.build()).isEqualTo(new URI("https://example.com/hello"));
 	}
 
+	@Test
+	void uriStringWhenEmptyShouldReturnRoot() throws URISyntaxException {
+		UriBuilder builder = this.builderFactory.uriString("");
+
+		URI builtUri = builder.build();
+		assertThat(builtUri).isEqualTo(new URI(this.rootUri));
+	}
+
 }