Add validation rule for empty sized containers

As it seems, GitHub passes unset values as empty strings, meaning that
they pass validation for the string type. To prevent empty strings from
making it past the verification feature, I've now added size validation
for whenever a value is a subclass of `collections.abc.Sized`.

Author: SebastiaanZ

github_status_embed/types.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import collections
 import dataclasses
 import enum
 import json
@@ -71,6 +72,8 @@ def from_arguments(cls, arguments: typing.Dict[str, str]) -> typing.Optional[Typ
                     value = _type[value.upper()]
                 else:
                     value = _type(value)
+                    if isinstance(value, collections.Sized) and len(value) == 0:
+                        raise ValueError
             except (ValueError, KeyError):
                 raise InvalidArgument(f"invalid value for `{attribute}`: {value}") from None
             else: