Throw ValueError when instantiating a VersionRange from None type #113

from univers.version_range import VersionRange 
>>> VersionRange.from_string(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "../univers/src/univers/version_range.py", line 102, in from_string
    vers = remove_spaces(vers)
  File "../univers/src/univers/utils.py", line 9, in remove_spaces
    return "".join(string.split())
AttributeError: 'NoneType' object has no attribute 'split'

from univers.version_range import VersionRange 
>>> VersionRange.from_string("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "../univers/src/univers/version_range.py", line 115, in from_string
    raise ValueError(f"{vers!r} must start with the 'vers:' URI scheme.")
ValueError: '' must start with the 'vers:' URI scheme.