fix: Handle unaliased fields breaking zmk_combos

Pydantic now returns None for field.alias if it doesn't
exist. This breaks `parse_config.zmk_combos` when you use
e.g. `draw_separate` which isn't aliased.

Fixes #77.

keymap_drawer/keymap.py

@@ -76,7 +76,7 @@ def normalize_fields(cls, spec_dict: dict) -> dict:
         """Normalize spec_dict so that each field uses its alias and key is parsed to LayoutKey."""
         for name, field in cls.model_fields.items():
             if name in spec_dict:
-                spec_dict[field.alias] = spec_dict.pop(name)
+                spec_dict[field.alias or name] = spec_dict.pop(name)
         if key_spec := spec_dict.get("k"):
             spec_dict["k"] = LayoutKey.from_key_spec(key_spec)
         return spec_dict