Fixing empty string in EvseSecurity config

[Pietfried]

Describe your changes

Checking if private key password is empty and passing std::nullopt to EvseSecurity ctor in this case.

This change is required, because even if the config parameter private_key_password is not configured, the constructor will pass an empty string instead of std::nullopt. This will cause private keys to be encrypted every time.

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• I read the contribution documentation and made sure that my changes meet its requirements

[SebaLukas]

@Pietfried clang-format failed

[james-ctc]

Note there are other places in the codebase where nullopt is converted to an empty string for passwords:
e.g.
EVP_PKEY_ptr evp_pkey(PEM_read_bio_PrivateKey(bio.get(), nullptr, nullptr, (void*)password.value_or("").c_str()));

Care is needed to ensure that existing keys that have been created with "" can still be used.