Add nullability constraints everywhere.

Change-Id: I80577b7784006070f29ce11af4e6fd87c10363bf
diff --git a/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java b/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java
index 5ecccc7..c402265 100644
--- a/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java
+++ b/src/main/java/eu/mulk/mulkcms2/benki/posts/Post.java
@@ -55,6 +55,7 @@
   public Integer id;
 
   @Column(name = "date", nullable = true)
+  @CheckForNull
   public OffsetDateTime date;
 
   @ManyToOne(fetch = FetchType.LAZY)