[Dependency Scanning] Make deprecated PCM arg libSwiftScan stubs return empty sets instead of nullptr

Recent change deprecated these methods and made the stubs return nullptr instead of empty sets, which will still trigger an error in a driver which pre-dates the change.

Author: artemcm

lib/Tooling/libSwiftScan/libSwiftScan.cpp

@@ -671,11 +671,11 @@ swiftscan_batch_scan_result_create(swiftscan_scanner_t scanner,
 }
 swiftscan_string_set_t *swiftscan_swift_textual_detail_get_extra_pcm_args(
    swiftscan_module_details_t details) {
-  return nullptr;
+  return swift::c_string_utils::create_empty_set();
 }
 swiftscan_string_set_t *
 swiftscan_clang_detail_get_captured_pcm_args(swiftscan_module_details_t details) {
-  return nullptr;
+  return swift::c_string_utils::create_empty_set();
 }
 swiftscan_batch_scan_input_t *swiftscan_batch_scan_input_create() {
   return nullptr;