title: Nullable foreign key on room
body: <p>I would know if that is possible to create a possible null reference on foreign key with room. </p>
  
  <p>For now my database structure is like this : </p>
  
  <pre class="lang-java prettyprint-override"><code>@Entity(tableName = "A")
  class A {
      @PrimaryKey(autoGenerate = true)
      public long id;
  }
  
  @Entity(tableName = "B")
  class B{
      @PrimaryKey(autoGenerate = true)
      public long id;
  }
  
  @Entity(tableName = "C", foreignKeys = {@ForeignKey(entity = A.class, parentColumns = "id", childColumns = "foreign_id_a"), @ForeignKey(entity = B.class, parentColumns = "id", childColumns = "foreign_id_b")})
  class C{
      public long id;
      public long foreign_id_a;
      public long foreign_id_b;   
  }
  </code></pre>
  
  <p>I would like to be able to insert the following objects : </p>
  
  <pre><code>C(id=1, foreign_id_a=1, foreign_id_b=1)
  C(id=1, foreign_id_a=null, foreign_id_b=1)
  C(id=1, foreign_id_a=1, foreign_id_b=null)
  </code></pre>
  
  <p>But the previous insert with null value give this error : 
  FOREIGN KEY constraint failed (Sqlite code 787 SQLITE_CONSTRAINT_FOREIGNKEY)</p>
  
  <p>Is there a way to make it possible ?</p>