Better error for named top-level union

[davydog187]

Top-level unions cannot be named (I think?). The current behavior needs to be adjusted accordingly to what the spec says.

Possibly helpful fastavro/fastavro#388

     test/encode_test.exs:393
     ** (FunctionClauseError) no function clause matching in AvroEx.Schema.cast/1

     The following arguments were given to AvroEx.Schema.cast/1:
     
         # 1
         %{"name" => "maybe_null", "type" => ["null", "string"]}
     
     Attempted function clauses (showing 10 out of 25):
     
         def cast(nil)
         def cast("null")
         def cast("boolean")
         def cast("int")
         def cast("long")
         def cast("float")
         def cast("double")
         def cast("bytes")
         def cast("string")
         def cast(%{"type" => nil} = data)
         ...
         (15 clauses not shown)
     
     code: schema = AvroEx.parse_schema!(~S({"type": ["null", "string"], "name": "maybe_null"}))
     stacktrace:

[davydog187]

Regardless, unions are supported at the top-level via ["null", "string"] in this case