Add @Nullable to ProblemDetail.setProperty argument

See gh-29868

spring-web/src/main/java/org/springframework/http/ProblemDetail.java

@@ -209,7 +209,7 @@ public class ProblemDetail {
 	 * @param value the property value
 	 * @see org.springframework.http.converter.json.ProblemDetailJacksonMixin
 	 */
-	public void setProperty(String name, Object value) {
+	public void setProperty(String name, @Nullable Object value) {
 		this.properties = (this.properties != null ? this.properties : new LinkedHashMap<>());
 		this.properties.put(name, value);
 	}

spring-web/src/main/java/org/springframework/http/converter/json/ProblemDetailJacksonMixin.java

@@ -23,6 +23,7 @@ import com.fasterxml.jackson.annotation.JsonAnySetter;
 import com.fasterxml.jackson.annotation.JsonInclude;
 
 import org.springframework.http.ProblemDetail;
+import org.springframework.lang.Nullable;
 
 import static com.fasterxml.jackson.annotation.JsonInclude.Include.NON_EMPTY;
 
@@ -46,7 +47,7 @@ import static com.fasterxml.jackson.annotation.JsonInclude.Include.NON_EMPTY;
 public interface ProblemDetailJacksonMixin {
 
 	@JsonAnySetter
-	void setProperty(String name, Object value);
+	void setProperty(String name, @Nullable Object value);
 
 	@JsonAnyGetter
 	Map<String, Object> getProperties();

spring-web/src/test/java/org/springframework/http/converter/json/ProblemDetailJacksonMixinTests.java

@@ -52,13 +52,15 @@ public class ProblemDetailJacksonMixinTests {
 	void writeCustomProperty() throws Exception {
 		ProblemDetail problemDetail = ProblemDetail.forStatusAndDetail(HttpStatus.BAD_REQUEST, "Missing header");
 		problemDetail.setProperty("host", "abc.org");
+		problemDetail.setProperty("user", null);
 
 		testWrite(problemDetail,
 				"{\"type\":\"about:blank\"," +
 						"\"title\":\"Bad Request\"," +
 						"\"status\":400," +
 						"\"detail\":\"Missing header\"," +
-						"\"host\":\"abc.org\"}");
+						"\"host\":\"abc.org\"," +
+						"\"user\":null}");
 	}
 
 	@Test
@@ -68,13 +70,15 @@ public class ProblemDetailJacksonMixinTests {
 						"\"title\":\"Bad Request\"," +
 						"\"status\":400," +
 						"\"detail\":\"Missing header\"," +
-						"\"host\":\"abc.org\"}", ProblemDetail.class);
+						"\"host\":\"abc.org\"," +
+						"\"user\":null}", ProblemDetail.class);
 
 		assertThat(problemDetail.getType()).isEqualTo(URI.create("about:blank"));
 		assertThat(problemDetail.getTitle()).isEqualTo("Bad Request");
 		assertThat(problemDetail.getStatus()).isEqualTo(HttpStatus.BAD_REQUEST.value());
 		assertThat(problemDetail.getDetail()).isEqualTo("Missing header");
 		assertThat(problemDetail.getProperties()).containsEntry("host", "abc.org");
+		assertThat(problemDetail.getProperties()).containsEntry("user", null);
 	}