Assure project_dir is always valid

Avoid case where project_dir could endup being None.

src/ansiblelint/cli.py

@@ -377,15 +377,18 @@ def get_config(arguments: List[str]) -> Namespace:
 
     options.rulesdirs = get_rules_dirs(options.rulesdir, options.use_default_rules)
 
-    if not options.project_dir:
+    if options.project_dir == ".":
         project_dir = os.path.dirname(
             os.path.abspath(
                 options.config_file or f"{guess_project_dir()}/.ansible-lint"
             )
         )
         options.project_dir = normpath(project_dir)
+    if not os.path.exists(options.project_dir):
+        raise RuntimeError(
+            f"Failed to determine a valid project_dir: {options.project_dir}"
+        )
 
-    # print(666, options.quiet, options.verbosity)
     # Compute final verbosity level by subtracting -q counter.
     options.verbosity -= options.quiet
     return config

src/ansiblelint/config.py

@@ -72,7 +72,7 @@
     mock_roles=[],
     loop_var_prefix=None,
     offline=False,
-    project_dir=None,
+    project_dir=".",  # default should be valid folder (do not use None here)
     extra_vars=None,
     enable_list=[],
     skip_action_validation=True,