title: Null check in formula column
body: <p>I am usable formula column in grails:</p>
  
  <pre><code> static mapping = {
          id generator: 'uuid'
          version false
          merchantTotal formula: "total_amount - penalty_fee"
  </code></pre>
  
  <p>Domain:</p>
  
  <pre><code>class Transaction {
      ....
      ............
      @Transient
      BigDecimal merchantTotal
  </code></pre>
  
  <p>The problem with above is that if any one row has "null" value in penalty_fee column then the total value of expression would be returned as null. I want to apply a null check or treat the null values as 0 in formula column.</p>