Fix TypeError when linter: or skip: keys are empty

Fix the error such as:

```
Traceback (most recent call last):
  File "/home/mgorny/miniforge3/bin/conda-smithy", line 10, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/mgorny/git/conda-smithy/conda_smithy/cli.py", line 767, in main
    args.subcommand_func(args)
  File "/home/mgorny/git/conda-smithy/conda_smithy/cli.py", line 645, in __call__
    lints, hints = linter.main(
                   ^^^^^^^^^^^^
  File "/home/mgorny/git/conda-smithy/conda_smithy/lint_recipe.py", line 733, in main
    results, hints = lintify_meta_yaml(
                     ^^^^^^^^^^^^^^^^^^
  File "/home/mgorny/git/conda-smithy/conda_smithy/lint_recipe.py", line 281, in lintify_meta_yaml
    if "lint_noarch_selectors" not in lints_to_skip:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: argument of type 'NoneType' is not iterable
```

when `conda-forge.yml` contains an empty key, e.g.:

```
linter:
  skip:
```

Noticed this accidentally while testing the other changes.

conda_smithy/lint_recipe.py

@@ -117,9 +117,9 @@ def lintify_meta_yaml(
     lints = []
     hints = []
     major_sections = list(meta.keys())
-    lints_to_skip = (
-        _get_forge_yaml(recipe_dir).get("linter", {}).get("skip", [])
-    )
+    lints_to_skip = (_get_forge_yaml(recipe_dir).get("linter") or {}).get(
+        "skip"
+    ) or []
 
     # If the recipe_dir exists (no guarantee within this function) , we can
     # find the meta.yaml within it.
@@ -487,9 +487,9 @@ def run_conda_forge_specific(
     hints,
     recipe_version: int = 0,
 ):
-    lints_to_skip = (
-        _get_forge_yaml(recipe_dir).get("linter", {}).get("skip", [])
-    )
+    lints_to_skip = (_get_forge_yaml(recipe_dir).get("linter") or {}).get(
+        "skip"
+    ) or []
 
     # Retrieve sections from meta
     package_section = get_section(