Fix NPE in FormHttpMessageConverter

Issue: SPR-10187

Author: rstoyanchev

org.springframework.web/src/main/java/org/springframework/http/converter/FormHttpMessageConverter.java

@@ -265,10 +265,12 @@ private void writeParts(OutputStream os, MultiValueMap<String, Object> parts, by
 		for (Map.Entry<String, List<Object>> entry : parts.entrySet()) {
 			String name = entry.getKey();
 			for (Object part : entry.getValue()) {
-				writeBoundary(boundary, os);
-				HttpEntity entity = getEntity(part);
-				writePart(name, entity, os);
-				writeNewLine(os);
+				if (part != null) {
+					writeBoundary(boundary, os);
+					HttpEntity entity = getEntity(part);
+					writePart(name, entity, os);
+					writeNewLine(os);
+				}
 			}
 		}
 	}

org.springframework.web/src/test/java/org/springframework/http/converter/FormHttpMessageConverterTests.java

@@ -112,6 +112,7 @@ public void writeMultipart() throws Exception {
 		parts.add("name 1", "value 1");
 		parts.add("name 2", "value 2+1");
 		parts.add("name 2", "value 2+2");
+		parts.add("name 3", null);
 
 		Resource logo = new ClassPathResource("/org/springframework/http/converter/logo.jpg");
 		parts.add("logo", logo);