Fix header-abi-diff skips diff when config.json is not found

Skip loading config.json when the config.json is not found so the
header-abi-diff will still perform abi diff.

Test: preform abi diff without config.json
Bug: 243328443
Change-Id: I92ebf7e7bfe6abb57bdefa211eaf604efaee517e

vndk/tools/header-checker/src/diff/header_abi_diff.cpp

@@ -213,7 +213,10 @@ bool ShouldEmitWarningMessage(CompatibilityStatusIR status) {
 int main(int argc, const char **argv) {
   llvm::cl::ParseCommandLineOptions(argc, argv, "header-checker");
 
-  ReadConfigFile(GetConfigFilePath(old_dump));
+  const std::string config_file_path = GetConfigFilePath(old_dump);
+  if (llvm::sys::fs::exists(config_file_path)) {
+    ReadConfigFile(config_file_path);
+  }
 
   std::set<std::string> ignored_symbols;
   if (llvm::sys::fs::exists(ignore_symbol_list)) {

vndk/tools/header-checker/src/utils/config_file.cpp

@@ -72,10 +72,7 @@ bool ConfigFile::Load(std::istream &istream) {
 
 bool ConfigFile::Load(const std::string &path) {
   std::ifstream stream(path);
-  if (stream.is_open()) {
-    return Load(stream);
-  }
-  return false;
+  return stream && Load(stream);
 }
 
 }  // namespace utils