Add unit tests for apply() method handling empty and slash-prefixed URIs

- Added tests for RootUriTemplateHandler.apply() covering:
  - URIs starting with '/'
  - Empty string
  - Blank string
  - Absolute URLs
  - Relative paths
- Ensures apply() behaves as expected in all edge cases

Signed-off-by: Hyunwoo Gu <realization0327@gmail.com>

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

@@ -32,6 +32,7 @@ import static org.mockito.Mockito.mock;
  * Tests for {@link RootUriBuilderFactory}.
  *
  * @author Scott Frederick
+ * @author Hyunwoo Gu
  */
 class RootUriBuilderFactoryTests {
 
@@ -43,4 +44,12 @@ class RootUriBuilderFactoryTests {
 		assertThat(builder.build()).isEqualTo(new URI("https://example.com/hello"));
 	}
 
+	@Test
+	void uriStringWhenEmptyShouldReturnRoot() throws URISyntaxException {
+		UriBuilderFactory builderFactory = new RootUriBuilderFactory("https://example.com",
+				mock(UriTemplateHandler.class));
+		UriBuilder builder = builderFactory.uriString("");
+		assertThat(builder.build()).isEqualTo(new URI("https://example.com"));
+	}
+
 }