fix: Allow empty paths in SecretsLoaderHelper

[simon-lemay-unity]

If a path was not provided, it would throw an exception while loading the (non-existent) file. But this will be the normal use case in live deployments, since servers and clients will have different files to load (e.g. the client will not be loading a private key).

This PR makes it so that not providing a path will just mean an empty string is used as the contents of the file. This should be functionally equivalent to not providing these values at all.

This PR also makes the paths empty by default. The previous defaults made sense in the context of the script in the test project that generates the files, but they are unlikely to be valuable to users otherwise. Not having any defaults also means we don't seem like we're taking position as to where certificates must be stored (having defaults pointing to the assets folder could have led users to think this was good or standard practice).

Changelog

N/A

Testing and Documentation

• No tests have been added.
• No documentation changes or additions were necessary.

[jeffreyrainy]

I don't mind the change. But, I think the exception was known and caught by

        private void Awake()
        {
            var serverSecrets = new ServerSecrets();

            try
            {
                serverSecrets.ServerCertificate = ServerCertificate;
            }
            catch (Exception exception)
            {
                Debug.Log(exception);
            }

[simon-lemay-unity]

@jeffreyrainy Yes, you're absolutely right. My PR description is inaccurate. I should have written that it only logs the exception (which is an order of magnitude less problematic than outright throwing). I guess my point was more that the behavior was non-optimal for a use case which should occur for a lot of deployments.