feat: Check if action run hasInput

[shrink]

I am trying to determine if an input has been provided in an action where an empty string is a valid value, unfortunately an empty string is treated the same as no input by getInput. I would ideally like to change getInput to distinguish between "no value" and "empty value" but that would be a breaking change: the introduction of hasInput is a safe middle-ground that enables this determination without any breaking changes.

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An alternative I considered was introducing a default to InputOptions which would help bring InputOptions and action.yml in line but I think that introduces more questions (should trimWhitespace be an option in action.yml?) so I went for the simpler option.

[shrink]

unfortunately, it turns out this doesn't work, because of the runner: https://github.com/actions/runner/blob/be9632302ceef50bfb36ea998cea9c94c75e5d4d/src/Runner.Worker/ActionRunner.cs#L189-L202

// Merge the default inputs from the definition
if (definition.Data?.Inputs != null)
{
    var manifestManager = HostContext.GetService<IActionManifestManager>();
    foreach (var input in definition.Data.Inputs)
    {
        string key = input.Key.AssertString("action input name").Value;
        validInputs.Add(key);
        if (!inputs.ContainsKey(key))
        {
            inputs[key] = manifestManager.EvaluateDefaultInput(ExecutionContext, key, input.Value);
        }
    }
}

I will try investigate further if it's possible to achieve this in a different way.

[shrink]

Acknowledged as a bug in actions/runner#924. I'll PR a fix for that and then bring this back.

Edit: #1176