Handle empty file input in HtmlUnitRequestBuilder

Prior to this commit, when using HtmlUnit with empty file input,
MockMvc's HtmlUnitRequestBuilder would throw a NullPointerException
when attempting to create a MockPart based on the null File.

This commit ensures that empty file input is converted to a MockPart
with a valid name but with a null filename, a null content type, and
empty content.

Closes gh-26799

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -373,8 +373,13 @@ final class HtmlUnitRequestBuilder implements RequestBuilder, Mergeable {
 		for (NameValuePair param : this.webRequest.getRequestParameters()) {
 			if (param instanceof KeyDataPair) {
 				KeyDataPair pair = (KeyDataPair) param;
-				MockPart part = new MockPart(pair.getName(), pair.getFile().getName(), readAllBytes(pair.getFile()));
+				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())) {
 					part.getHeaders().setContentType(MediaType.valueOf(pair.getMimeType()));
+				}
 				request.addPart(part);
 			}
 			else {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -18,6 +18,7 @@ package org.springframework.test.web.servlet.htmlunit;
 
 import java.net.MalformedURLException;
 import java.net.URL;
+import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Collections;
@@ -423,8 +424,7 @@ public class HtmlUnitRequestBuilderTests {
 	}
 
 	@Test // gh-24926
-	public void buildRequestParameterMapViaWebRequestDotSetFileToUploadAsParameter() throws Exception {
-
+	public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithFileToUploadAsParameter() throws Exception {
 		webRequest.setRequestParameters(Collections.singletonList(
 				new KeyDataPair("key",
 						new ClassPathResource("org/springframework/test/web/htmlunit/test.txt").getFile(),
@@ -432,7 +432,7 @@ public class HtmlUnitRequestBuilderTests {
 
 		MockHttpServletRequest actualRequest = requestBuilder.buildRequest(servletContext);
 
-		assertThat(actualRequest.getParts().size()).isEqualTo(1);
+		assertThat(actualRequest.getParts()).hasSize(1);
 		Part part = actualRequest.getPart("key");
 		assertThat(part).isNotNull();
 		assertThat(part.getName()).isEqualTo("key");
@@ -441,6 +441,21 @@ public class HtmlUnitRequestBuilderTests {
 		assertThat(part.getContentType()).isEqualTo(MimeType.TEXT_PLAIN);
 	}
 
+	@Test // gh-26799
+	public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithNullFileToUploadAsParameter() throws Exception {
+		webRequest.setRequestParameters(Collections.singletonList(new KeyDataPair("key", null, null, null, (Charset) null)));
+
+		MockHttpServletRequest actualRequest = requestBuilder.buildRequest(servletContext);
+
+		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();
+	}
+
 	@Test
 	public void buildRequestParameterMapFromSingleQueryParam() throws Exception {
 		webRequest.setUrl(new URL("https://example.com/example/?name=value"));