Nullability refinements in spring-webmvc

Includes revision of web.servlet.tags.form for non-null conventions.

Issue: SPR-15540

spring-context/src/main/java/org/springframework/ui/ModelMap.java

@@ -51,7 +51,7 @@ public class ModelMap extends LinkedHashMap<String, Object> {
 	 * under the supplied name.
 	 * @see #addAttribute(String, Object)
 	 */
-	public ModelMap(String attributeName, Object attributeValue) {
+	public ModelMap(String attributeName, @Nullable Object attributeValue) {
 		addAttribute(attributeName, attributeValue);
 	}