Fix None passed to LengthValidator

It is valid to pass None as a default value to the validator. It should
just return ValueError, which it will do with this change via the inner
validator.

Fixes #17961

Ideally we'd add type annotations, but with the current structure and
instantiation order it seems impossible to do this in a meaningful way.

If we refactored the validators to be functions, and validators
functions were registered per parameter type we could have narrow types on the
validator signature.

lib/galaxy/tools/parameters/validation.py

@@ -186,7 +186,9 @@ def __init__(self, message, length_min, length_max, negate):
         super().__init__(message, range_min=length_min, range_max=length_max, negate=negate)
 
     def validate(self, value, trans=None):
-        super().validate(len(value), trans)
+        if value is None:
+            raise ValueError("No value provided")
+        super().validate(len(value) if value else 0, trans)
 
 
 class DatasetOkValidator(Validator):

test/unit/app/tools/test_parameter_validation.py

@@ -186,6 +186,14 @@ def test_LengthValidator(self):
             p.validate("bar")
         p.validate("f")
         p.validate("foobarbaz")
+        p = self._parameter_for(
+            xml="""
+<param name="blah" type="text" optional="false">
+    <validator type="length" min="2" max="8"/>
+</param>"""
+        )
+        with self.assertRaisesRegex(ValueError, "No value provided"):
+            p.validate(None)
 
     def test_InRangeValidator(self):
         p = self._parameter_for(