Allow Falsey values for IsNumber validator

We should use Required() instead to ensure data is present, if that's the
desired behavior

tests/forms/test_validators.py

@@ -11,7 +11,7 @@ class TestIsNumber:
         dummy_field.data = valid
         validator(dummy_form, dummy_field)
 
-    @pytest.mark.parametrize("invalid", ["12.1", "two", "", None])
+    @pytest.mark.parametrize("invalid", ["12.1", "two"])
     def test_IsNumber_rejects_anything_else(self, invalid, dummy_form, dummy_field):
         validator = IsNumber()
         dummy_field.data = invalid