Fix null parameterName issue in content negotiation

After this change ParameterContentNegotiationStrategy no longer allows
a null parameter name, ContentNegotiationManagerFactoryBean also
requires it.

Issue: SPR-10139

spring-web/src/main/java/org/springframework/web/accept/ContentNegotiationManagerFactoryBean.java

@@ -27,6 +27,7 @@ import javax.servlet.ServletContext;
 import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.http.MediaType;
+import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
 import org.springframework.web.context.ServletContextAware;
 
@@ -55,7 +56,7 @@ public class ContentNegotiationManagerFactoryBean
 
 	private Boolean useJaf;
 
-	private String parameterName;
+	private String parameterName = "format";
 
 	private MediaType defaultContentType;
 
@@ -126,6 +127,7 @@ public class ContentNegotiationManagerFactoryBean
 	 * <p>The default parameter name is {@code "format"}.
 	 */
 	public void setParameterName(String parameterName) {
+		Assert.notNull(parameterName, "parameterName is required");
 		this.parameterName = parameterName;
 	}
 

spring-web/src/main/java/org/springframework/web/accept/ParameterContentNegotiationStrategy.java

@@ -44,7 +44,6 @@ public class ParameterContentNegotiationStrategy extends AbstractMappingContentN
 	 */
 	public ParameterContentNegotiationStrategy(Map<String, MediaType> mediaTypes) {
 		super(mediaTypes);
-		Assert.notEmpty(mediaTypes, "Cannot look up media types without any mappings");
 	}
 
 	/**
@@ -52,6 +51,7 @@ public class ParameterContentNegotiationStrategy extends AbstractMappingContentN
 	 * <p>The default parameter name is {@code format}.
 	 */
 	public void setParameterName(String parameterName) {
+		Assert.notNull(parameterName, "parameterName is required");
 		this.parameterName = parameterName;
 	}
 

spring-web/src/test/java/org/springframework/web/accept/ContentNegotiationManagerFactoryBeanTests.java

@@ -88,7 +88,6 @@ public class ContentNegotiationManagerFactoryBeanTests {
 	@Test
 	public void favorParameter() throws Exception {
 		this.factoryBean.setFavorParameter(true);
-		this.factoryBean.setParameterName("f");
 
 		Properties mediaTypes = new Properties();
 		mediaTypes.put("json", MediaType.APPLICATION_JSON_VALUE);
@@ -98,7 +97,7 @@ public class ContentNegotiationManagerFactoryBeanTests {
 		ContentNegotiationManager manager = this.factoryBean.getObject();
 
 		this.servletRequest.setRequestURI("/flower");
-		this.servletRequest.addParameter("f", "json");
+		this.servletRequest.addParameter("format", "json");
 
 		assertEquals(Arrays.asList(MediaType.APPLICATION_JSON), manager.resolveMediaTypes(this.webRequest));
 	}