Fix cryptic error for empty arrays in JSON config file

[sebastiangrimberg]

Previously, an (incorrect) configuration file array specified as "Attributes": [] would lead to an error like:

Verification failed: (ec == std::errc()) is false:
 --> Invalid integer conversion in range expansion!
 ... in function: palace::{anonymous}::PreprocessFile(const char*)::<lambda(std::string_view)>
 ... in file: /apps/palace/palace/palace/utils/iodata.cpp:77

This case is now handled and checked for in the JSON Schema during --dry-run.