fix: initialise default tsa_uri in c2pa::Signer constructor with an empty string

[kostiantynperehuda]

Constructing a std::string from a nullptr is undefined behavior and has been removed in C++23.

This fix ensures well-defined behavior and allows the project to build successfully when using the C++23 standard.

[yogeshbdeshpande]

Please note that the ADOBE CLA was signed from Arm as an organisation so this contribution needs to proceed.

Also, request add suitable reviewer to the PR and please provide review comments and approve.

Thanks for all help.

If any questions: Please contact [email protected]

[yogeshbdeshpande]

@gpeacock : Please add suitable reviewers!

[tmathern]

Added @gpeacock and @dyro as reviewers.

[dyro]

I'd prefer to change this to the following, if we are going to make a change to the API:

Header

Signer::Signer(const string &alg, const string &sign_cert, const string &private_key, optional<std::string> tsa_uri)

Implementation

// implementation then would be something like this:
Signer::Signer(const string &alg, const string &sign_cert, const string &private_key,
               optional<std::string> tsa_uri) {
    auto info = C2paSignerInfo{
        alg.c_str(),
        sign_cert.c_str(),
        private_key.c_str(),
        tsa_uri.has_value() ? tsa_uri->c_str() : nullptr
    };
    signer = c2pa_signer_from_info(&info);
}

Since we are already changing the API, let's make it more semantically correct. Also, using a string as the default changes the behavior of the API. Using a zero length string would mean we are passing a non-nullptr to the Rust code, which may change the behavior of the rust sdk. Lemme know what you think!

[gpeacock]

We already include in the headers and use it in other apis, so this is the right approach here.
FYI, we are a little inconsistent, usually just referring to optional instead of std:optional in the source.