Ion CLI fails to parse null symbol tables

[popematt]

The spec says that these two symbol tables are equivalent:

$ion_symbol_table::{}
$ion_symbol_table::null.struct

But Ion CLI fails to parse a null symbol table:

❯ ion inspect --hex="E0 01 00 EA E3 81 83 DF"
Error: input: E0 01 00 EA E3 81 83 DF

Caused by:
    0: expected a(n) struct but found a null.struct
    1: expected a(n) struct but found a null.struct
┌──────────────┬──────────────┬─────────────────────────┬──────────────────────┐
│    Offset    │    Length    │       Binary Ion        │       Text Ion       │
├──────────────┼──────────────┼─────────────────────────┼──────────────────────┘
│            0 │            4 │ e0 01 00 ea             │ $ion_1_0 // Version marker

❯ echo '$ion_symbol_table::null.struct' | ion cat
Error: expected a(n) struct but found a null.struct

Caused by:
    expected a(n) struct but found a null.struct

The empty struct case is handled correctly.