Check for NULL APIs returned by sai_api_query() and handle gracefully instead of crashing.

[chrispsommers]

Fixes #1535. I modified the code-generator template to check for null SAI function pointers before calling, and throwing an exception with helpful message on the console.

Replaces #1536 which had some rebasing conflicts.

The changed file of interest is actually just meta/templates/sai_rpc_server_functions.tt.

Example console trace when calling switch_api->get_switch_attribute() which isn't implemented and has a null pointer. The process does not crash anymore.

Starting SAI RPC server on port 9092
NULL ptr: switch_api->get_switch_attribute
To understand how this works, the template inserts the following code (as an example) automatically:

if (switch_api->get_switch_stats == (void *)0) {
    std::cerr << "NULL ptr: switch_api->get_switch_stats" << std::endl;

  sai_thrift_exception e;
  e.status = SAI_STATUS_NOT_IMPLEMENTED;
  throw e;
}

right before making any API call such as:

status = switch_api->get_switch_stats(switch_id, number_of_counters, sai_counter_ids, sai_counters);