Adapt empty path to "/" in MockMvc

Closes gh-29933

spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -149,7 +149,8 @@ public class MockHttpServletRequestBuilder
 		Assert.notNull(url, "'url' must not be null");
 		Assert.isTrue(url.isEmpty() || url.startsWith("/") || url.startsWith("http://") || url.startsWith("https://"),
 				() -> "'url' should start with a path or be a complete HTTP URL: " + url);
-		return UriComponentsBuilder.fromUriString(url).buildAndExpand(vars).encode().toUri();
+		String uriString = (url.isEmpty() ? "/" : url);
+		return UriComponentsBuilder.fromUriString(uriString).buildAndExpand(vars).encode().toUri();
 	}
 
 	/**

spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java

@@ -167,15 +167,15 @@ class MockHttpServletRequestBuilderTests {
 		assertThat(request.getPathInfo()).isNull();
 	}
 
-	@Test // gh-28823
+	@Test // gh-28823, gh-29933
 	void emptyPath() {
 		this.builder = new MockHttpServletRequestBuilder(GET, "");
 		MockHttpServletRequest request = this.builder.buildRequest(this.servletContext);
 
-		assertThat(request.getRequestURI()).isEqualTo("");
+		assertThat(request.getRequestURI()).isEqualTo("/");
 		assertThat(request.getContextPath()).isEqualTo("");
 		assertThat(request.getServletPath()).isEqualTo("");
-		assertThat(request.getPathInfo()).isNull();
+		assertThat(request.getPathInfo()).isEqualTo("/");
 	}
 
 	@Test // SPR-16453