title: Custom serialization of null/initial values in ABAP using `CALL TRANSFORMATION`
body: <p>You can serialize ABAP structures to JSON using the <code>CALL TRANSFORMATION</code> command. Is there any possibility to serialize <code>null</code> values for initial fields or ignore initial fields?</p>
  
  <pre><code>TYPES: BEGIN OF t_my_type,
           foo TYPE string,
           bar TYPE string,
         END OF t_my_type.
  
  DATA ls_structure TYPE t_my_type.
  ls_structure-foo = 'some value'.
  ASSERT ls_structure-bar IS INITIAL.
  DATA(lo_json_writer) = cl_sxml_string_writer=&gt;create( type = if_sxml=&gt;co_xt_json ).
  CALL TRANSFORMATION id SOURCE result = ls_structure RESULT XML lo_json_writer.
  DATA(lv_json_xstring) = lo_json_writer-&gt;get_output( ).
  WRITE cl_abap_codepage=&gt;convert_from( source = lv_json_xstring ).
  </code></pre>
  
  <p>The result of this minimal working example is:</p>
  
  
  
  <pre><code>{
    "RESULT": {
      "FOO": "some value",
      "BAR": ""
    }
  }
  </code></pre>
  
  <p>In other languages (that need to process the json exported from SAP) an empty string <code>""</code> and <code>null</code> are not the same and do require additional processing/handling I'd like to avoid. I want a result like this</p>
  
  <pre><code>{
    "RESULT": {
      "FOO": "some value",
      "BAR": null
    }
  }
  </code></pre>
  
  <p>or ignore the initial value like this:</p>
  
  <pre><code>{
    "RESULT": {
      "FOO": "some value"
    }
  }
  </code></pre>
  
  <p>Can I pass some additional rules to <code>CALL TRANSFORMATION</code> on how to handle <code>null</code>/initial values?</p>