Remove null params in HttpParams

You should filter them client side using JavaScript's delete (MDN) keyword if they have a value of undefined (which is different than the key not being in the object at all):

if (queryObject.secsymb === undefined) {
  delete queryObject.secsymb;
}

and then use a default method parameter in your controller method:

GetExposureRunsData(int id, string fund, SecType sectype, string secsymb = null)

If you ensure that the query string parameter is not included then your parameter will default to null