The function takes a string input and returns the textual representation of the individual numerical digits found in the input. For inputs without any numerical digits, it returns an empty string.

[CODE]: 
```python
def approximate_function(input_str):
    num_to_text = {
        '0': 'zero', '1': 'one', '2': 'two', '3': 'three', '4': 'four',
        '5': 'five', '6': 'six', '7': 'seven', '8': 'eight', '9': 'nine'
    }
    output = ' '.join(num_to_text.get(char, '') for char in input_str).strip()
    return output
```

Please test the described function and code with more input strings if needed.