Serialization issue when using `missing`

The following round-trip serialization has an error when reading the data back in. The error is `ERROR: MethodError: no method matching Union{Missing, String}(::String)`.

```julia
using Catlab

@present SimpleSch(FreeSchema) begin
X::Ob
Attr1::AttrType
attr::Attr(X,Attr1)
end

@acset_type SimpleData(SimpleSch)

simple_data = @acset SimpleData{Union{String,Missing}} begin
X=3
attr=["a",missing,"c"]
end

write_json_acset_schema(SimpleSch, "./simplesch.json")
write_json_acset(simple_data, "./simpledata.json")

SimpleSch1 = read_json_acset_schema("./simplesch.json")

@acset_type SimpleData1(SimpleSch1)

simple_data1 = read_json_acset(SimpleData1{Union{String,Missing}}, "./simpledata.json")
```

On versions
```
[227ef7b5] ACSets v0.2.13
[134e5e36] Catlab v0.16.5
```