Fix returning an empty task_handle from task_group task

[kboyarinov]

Description

Task Group Dynamic Dependencies broke the case when an empty task_handle is bypassed.
Current documentation do not explicitly allow it, but it is needed to implement constructions like below:

class group_task {
    tbb::task_handle operator()() const {
        if (range-too-small) {
            // Do serial work
            return tbb::task_handle{}; // nothing to bypass, but the operator signature forces to return something
        } else {
            // Split work and run in parallel
            tbb::task_handle left = tg.defer(left-subtask);
            tbb::task_handle right = tg.defer(right-subtask);
            
            // Submit the right part
            tg.run(std::move(right);
            
            // Bypass the left part
            return left;
        }
    }
};

Fixes # - issue number(s) if exists

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• bug fix - change that fixes an issue
• new feature - change that adds functionality
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• documentation - documentation update

Tests

• added - required for new features and some bug fixes
• not needed

Documentation

• updated in # - add PR number
• needs to be updated
• not needed

Breaks backward compatibility

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• No
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[vossmjp]

Looks good to me