title: How to Migrate Not Null table column into Null in Android Room database
body: <p>I'm new to android room library. I need to migrate a Not Null column to Null,
  But room migration only allow ADD or RENAME in ALTER table query. How do execute a column migration query?</p>
  
  <pre><code>@Entity(tableName = "vehicle_detail")
  data class VehicleDetailsEntity(
      @PrimaryKey(autoGenerate = true)
      val vehicleClientId: Long = 0,
      val vehicleId: String,
      val updatedOn: Date,
      val updatedBy: String
  )
  </code></pre>
  
  <p>I need to change table structure into</p>
  
  <pre><code>@Entity(tableName = "vehicle_detail")
  data class VehicleDetailsEntity(
      @PrimaryKey(autoGenerate = true)
      val vehicleClientId: Long = 0,
      val vehicleId: String,
      val updatedOn: Date?,
      val updatedBy: String?
  )
  </code></pre>
  
  <p>java.lang.IllegalStateException: Room cannot verify the data integrity. Looks like you've changed schema but forgot to update the version number. You can simply fix this by increasing the version number.</p>