Serialize location information to the following JSON format:

"position":"<position>",
"line":"<line>",
"column":"<column>",

@param {SourceLocation} location The location to serialize, may be undefined.
function serializeLocationFields (location, content) {
  if (!location) {
    return;
  }
  content.position = location.position;
  var line = location.line;
  if (!IS_UNDEFINED(line)) {
    content.line = line;
  }
  var column = location.column;
  if (!IS_UNDEFINED(column)) {
    content.column = column;
  }
}