Empty string should be considered a legal import ID

[zslayton]

Interesting corner case here: given the intentionally open-ended language in the spec, I think it's possible for the empty string to be a valid import id. parentImportId can be set to "" on line 138; if that happens, attempting to import a child with id="" would cause an error.

To understand this better we'd need to look at all the invocation sites of this class's constructor to see where/why schemaId is set to null.

Originally posted by @zslayton in #157 (comment)

[popematt]

Do you think this is an issue in the implementation, or a mistake in the spec? With the existing AuthorityFilesystem, you can't actually load any schema with an id of "", so it seems unlikely that the spec is intentionally allowing an empty string as a schema id.

[zslayton]

Without diving into it too deeply, I'd guess it's an issue in the implementation. Ion and its related tools don't usually forbid the empty string as a value. For example: symbol table names can be the empty string and struct field names can be the empty string.