Mint.HTTP1.Request private fn validate_header_value throws Erlang error: {:bad_generator, nil} when value is nil

[marinac-dev]

If the value of header is nil it will just throw error.

** (ErlangError) Erlang error: {:bad_generator, nil}
    (mint 1.5.1) lib/mint/http1/request.ex:85: Mint.HTTP1.Request."-validate_header_value!/2-lc$^0/1-0-"/3
    (mint 1.5.1) lib/mint/http1/request.ex:85: Mint.HTTP1.Request.validate_header_value!/2
    (mint 1.5.1) lib/mint/http1/request.ex:27: anonymous fn/2 in Mint.HTT
    # . . . Stack strace

I think I could make a bug reproducing repo in free time, but what caused this in my case is a dependency. In dependency, there is macro @api_key Application.compile_env(:my_app, :api_key) (yes I know I shouldn't use that, I will change it) which requires setting it in somewhere in config.exs but if I don't do that header value becomes nil and bum bug. :)

In my config, I use System.get_env to fetch header value I am using Tesla with Finch adapter

[ericmj]

We do not accept nil in the headers as seen in the typespec: https://hexdocs.pm/mint/Mint.Types.html#t:headers/0. Handling nil would mean we have to make the decision if the header should be removed or the value should be "". This decision will be different for different applications so mint cannot make it.

If you are using System.get_env it takes a second argument where you can give a default value that isn't nil.