Add missing type constraints to `findBy` repository methods.

Closes #5050

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/QuerydslMongoPredicateExecutor.java

@@ -22,8 +22,8 @@ import java.util.function.Function;
 import java.util.stream.Stream;
 
 import org.bson.Document;
-
 import org.jspecify.annotations.Nullable;
+
 import org.springframework.dao.IncorrectResultSizeDataAccessException;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.Pageable;
@@ -165,7 +165,7 @@ public class QuerydslMongoPredicateExecutor<T> extends QuerydslPredicateExecutor
 
 	@Override
 	@SuppressWarnings("unchecked")
-	public <S extends T, R> R findBy(Predicate predicate,
+	public <S extends T, R extends @Nullable Object> R findBy(Predicate predicate,
 			Function<FluentQuery.FetchableFluentQuery<S>, R> queryFunction) {
 
 		Assert.notNull(predicate, "Predicate must not be null");

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SimpleMongoRepository.java

@@ -341,7 +341,7 @@ public class SimpleMongoRepository<T, ID> implements MongoRepository<T, ID> {
 	}
 
 	@Override
-	public <S extends T, R> R findBy(Example<S> example,
+	public <S extends T, R extends @Nullable Object> R findBy(Example<S> example,
 			Function<org.springframework.data.repository.query.FluentQuery.FetchableFluentQuery<S>, R> queryFunction) {
 
 		Assert.notNull(example, "Sample must not be null");