Some modules or module versions don't have a metadata.json file, but we
might still want to use the load_module_metadata function on them. The
lack of a file can still give us important information. For example, it
might tell us that the version of the module installed is "very old"
even if we can't read the version number directly. This patch adds a
parameter to let the user specify if an empty file is acceptable. To
preserve backwards compatibility it does not change the current default
behavior, which is to raise an error if metadata.json does not exist.
