Handle NoneType in VersionRange.from_string (#115)

* Handle NoneType in VersionRange.from_string

Signed-off-by: Keshav Priyadarshi <git@keshav.space>

* Address review comments

Signed-off-by: Keshav Priyadarshi <git@keshav.space>

---------

Signed-off-by: Keshav Priyadarshi <git@keshav.space>

Author: keshav-space

src/univers/version_range.py

@@ -98,6 +98,11 @@ def from_string(cls, vers, simplify=False, validate=False):
         Return a VersionRange built from a ``vers`` version range spec string,
         such as "vers:npm/1.2.3,>=2.0.0"
         """
+        if not vers or not isinstance(vers, str) or not vers.strip():
+            raise ValueError(
+                f"{vers!r} is not a valid argument, a valid ``vers`` string argument is required."
+            )
+
         # Spaces are not significant and removed in a canonical form.
         vers = remove_spaces(vers)