Reconsider handling of no-text inputs to make_string / make_symbol

[toddjonker]

The spec currently states that make_string

Produces a non-null, unannotated string containing the concatenated content produced by the arguments. Nulls and annotations are discarded.

There's still some discussion, eg at amazon-ion/ion-tests#100 (comment)

The spec also ignores $0 and absent shared symbols. See also #324

BTW the case that gives me the most pause is with one null, eg (make_symbol null.string) which one might expect to return null.symbol not "". That's fairly useless for E-expression invocation (you could just write the desired null directly), but in templates it enables coercing any string to a symbol.

[popematt]

Completed in #342. Passing nulls and symbols with unknown text to make_string or make_symbol must cause an error to be raised.