Handle empty file input in HtmlUnitRequestBuilder

This commit revises the fix submitted in 959e6d1 by ensuring that
empty file input is converted to a MockPart with the supplied name, an
empty filename, "application/octet-stream" as the content type, and
empty content.

This aligns with the behavior of Servlet containers, as tested with the
interaction between Firefox and a standard Servlet running in a Jetty
Servlet container.

Closes gh-26799

Author: sbrannen

spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilder.java

@@ -374,12 +374,15 @@ private void params(MockHttpServletRequest request, UriComponents uriComponents)
 			if (param instanceof KeyDataPair) {
 				KeyDataPair pair = (KeyDataPair) param;
 				File file = pair.getFile();
-				MockPart part = (file != null ?
-						new MockPart(pair.getName(), file.getName(), readAllBytes(file)) :
-						new MockPart(pair.getName(), null));
-				if (StringUtils.hasLength(pair.getMimeType())) {
+				MockPart part;
+				if (file != null) {
+					part = new MockPart(pair.getName(), file.getName(), readAllBytes(file));
 					part.getHeaders().setContentType(MediaType.valueOf(pair.getMimeType()));
 				}
+				else { // mimic empty file upload
+					part = new MockPart(pair.getName(), "", null);
+					part.getHeaders().setContentType(MediaType.APPLICATION_OCTET_STREAM);
+				}
 				request.addPart(part);
 			}
 			else {

spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilderTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.test.web.servlet.htmlunit;
 
+import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.nio.charset.Charset;
@@ -53,6 +54,7 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
+import static org.assertj.core.api.SoftAssertions.assertSoftly;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
 
 /**
@@ -449,11 +451,20 @@ public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithNull
 
 		assertThat(actualRequest.getParts()).hasSize(1);
 		Part part = actualRequest.getPart("key");
-		assertThat(part).isNotNull();
-		assertThat(part.getName()).isEqualTo("key");
-		assertThat(IOUtils.toString(part.getInputStream(), StandardCharsets.UTF_8)).isEqualTo("");
-		assertThat(part.getSubmittedFileName()).isNull();
-		assertThat(part.getContentType()).isNull();
+
+		assertSoftly(softly -> {
+			softly.assertThat(part).isNotNull();
+			softly.assertThat(part.getName()).as("name").isEqualTo("key");
+			softly.assertThat(part.getSize()).as("size").isEqualTo(0);
+			try {
+				softly.assertThat(part.getInputStream()).isEmpty();
+			}
+			catch (IOException ex) {
+				softly.fail("failed to get InputStream", ex);
+			}
+			softly.assertThat(part.getSubmittedFileName()).as("filename").isEqualTo("");
+			softly.assertThat(part.getContentType()).as("content-type").isEqualTo("application/octet-stream");
+		});
 	}
 
 	@Test