[PHP] Fix path munging when package is absent (#33126)

Cloning https://github.com/grpc/grpc/pull/33124 to workaround a CLA bot
issue.

src/compiler/php_generator_helpers.h

@@ -60,8 +60,10 @@ inline std::string GetPHPServiceFilename(
           << grpc_generator::CapitalizeFirstLetter(tokens[i]);
     }
   }
-  return oss.str() + "/" +
-         GetPHPServiceClassname(service, class_suffix, is_server) + ".php";
+  std::string path = oss.str();
+  if (!path.empty()) path += "/";
+  path += GetPHPServiceClassname(service, class_suffix, is_server) + ".php";
+  return path;
 }
 
 // Get leading or trailing comments in a string. Comment lines start with "// ".