bert_validator's inability to work with type `string()`

[rdimitrovbg]

During compilation using rebar3 (rebar 3.10.0 on Erlang/OTP 20 Erts 9.0) the following error was raised

Error in parse transform 'bert_validator': {function_clause,
                                            [{bert_validator,guard,
                                              [{union,
                                                [{type,25,string,[]},
                                                 {type,25,binary,[]}]},
                                               "Name",
                                               {form,
                                                [10,118,97,108,105,100,97,116,
                                                 101,40,68,32,61,32,35,39,119,
                                                 114,105,116,101,114,39,123,
                                                 105,100,32,61,32,73,100,44,
                                                 32,99,111,117,110,116,32,61 ...