fix(parser,selection): correct None handling and Path type checks

- Ensure required argument validation treats only `None` as missing
  instead of falsy values (e.g., 0, False, empty string)
- Guard SelectionAction default resolution against `None` results
- Replace direct `type == Path` checks with `issubclass(..., Path)`
  for proper handling of Path subclasses across suggestions logic

Improves correctness in argument parsing and selection defaults,
aligning with Falyx’s explicit and predictable behavior goals.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "falyx"
-version = "0.1.86"
+version = "0.1.87"
 description = "Reliable and introspectable async CLI action framework."
 authors = ["Roland Thomas Jr <roland@rtj.dev>"]
 license = "MIT"