title: Django REST replace empty to default
body: <p>I have a Django REST API set up and it works properly for valid incoming requests. In some of the requests, some of the fields are empty. Is there a way to provide default replacement values for those empty fields in the serializer, so that they would pass the validation test? For example, I have the following serializer:</p>
  
  <pre><code>class SearchRequestSerializer(serializers.ModelSerializer):
  
    myfield1 = serializers.DecimalField(max_digits=10, decimal_places=2, coerce_to_string=False, default=0, required=False, allow_null=True)
    class Meta:
      model = SearchRequest
      fields = ('myfield0', 'myfield1')
  </code></pre>
  
  <p>myfield1 is sometimes not provided. As shown above, I tried to default it to 0, but still getting</p>
  
  <pre><code>"myfield1":["A valid number is required."]
  </code></pre>
  
  <p>I don't know if it has any impact, but my requests are arrays and I'm using the serializer with the option many=True.</p>
  
  <p>An example incomplete request would look like:</p>
  
  <pre><code>[{"myfield0":3, "myfield1":""}, {"myfield0":4, "myfield1":5}]
  </code></pre>