text: Title: [BUG-REPORT] Issue converting object to string
  Body: **Description**
  Trying to convert an "object" column to string fails.
  
  Example code:
  ```python
  import vaex
  import numpy as np
  
  if __name__ == "__main__":
      arr = np.array([123, "test", None], dtype=object)
      df = vaex.from_arrays(test=arr)
      df['test'] = df['test'].astype('str')
      print(df.head())
  ```
  
  Exception:
  ```python
  TypeError: to_string(): incompatible function arguments. The following argument types are supported:
      1. (arg0: numpy.ndarray[float32]) -> vaex.superstrings.StringList64
      2. (arg0: numpy.ndarray[float64]) -> vaex.superstrings.StringList64
      3. (arg0: numpy.ndarray[int64]) -> vaex.superstrings.StringList64
      4. (arg0: numpy.ndarray[int32]) -> vaex.superstrings.StringList64
      5. (arg0: numpy.ndarray[int16]) -> vaex.superstrings.StringList64
      6. (arg0: numpy.ndarray[int8]) -> vaex.superstrings.StringList64
      7. (arg0: numpy.ndarray[uint64]) -> vaex.superstrings.StringList64
      8. (arg0: numpy.ndarray[uint32]) -> vaex.superstrings.StringList64
      9. (arg0: numpy.ndarray[uint16]) -> vaex.superstrings.StringList64
      10. (arg0: numpy.ndarray[uint8]) -> vaex.superstrings.StringList64
      11. (arg0: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      12. (arg0: numpy.ndarray[float32], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      13. (arg0: numpy.ndarray[float64], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      14. (arg0: numpy.ndarray[int64], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      15. (arg0: numpy.ndarray[int32], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      16. (arg0: numpy.ndarray[int16], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      17. (arg0: numpy.ndarray[int8], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      18. (arg0: numpy.ndarray[uint64], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      19. (arg0: numpy.ndarray[uint32], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      20. (arg0: numpy.ndarray[uint16], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      21. (arg0: numpy.ndarray[uint8], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
      22. (arg0: numpy.ndarray[bool], arg1: numpy.ndarray[bool]) -> vaex.superstrings.StringList64
  
  Invoked with: array([123], dtype=object)
  
  Process finished with exit code 0
  ```
  
  
  **Software information**
   - Vaex version (`import vaex; vaex.__version__)`: 
    ```
  {'vaex': '4.5.0', 'vaex-core': '4.6.0a5', 'vaex-viz': '0.5.0', 'vaex-hdf5': '0.10.0', 'vaex-server': '0.6.1', 'vaex-astro': '0.9.0', 'vaex-jupyter': '0.6.0', 'vaex-ml': '0.14.0', 'vaex-arrow': '0.4.2'}
  ```
   - Vaex was installed via: pip
   - OS: Ubuntu 20.04
  
  **Additional information**
  This makes it impossible to perform an export to HDF5 or other operations that do not support the object type. This is especially blocking when loading CSV files.
  Should be relevant to #1568.
  