Add nullability annotations to module/spring-boot-data-r2dbc

See gh-46587

Author: mhalbritter

module/spring-boot-data-r2dbc/src/main/java/org/springframework/boot/data/r2dbc/autoconfigure/package-info.java

@@ -17,4 +17,7 @@
 /**
  * Auto-Configuration for Spring Data R2DBC.
  */
+@NullMarked
 package org.springframework.boot.data.r2dbc.autoconfigure;
+
+import org.jspecify.annotations.NullMarked;