Add type hinting to on_skip and fix unhandled potential runtime error

In adding type hinting to `on_skip`, and `do_skip`, a potential runtime
error was uncovered wherein we were accessing `self.skip_cause.status` in
`on_skip` when the `skip_cause` can be `None`. On that edge case, a
runtime error would be raised given how we were attempting to access
the `status` property. We now handle this edge case, and we only discovered
it because of the additional typing.

core/dbt/task/base.py

@@ -449,7 +449,7 @@ def _skip_caused_by_ephemeral_failure(self) -> bool:
             return False
         return self.skip_cause.node.is_ephemeral_model
 
-    def on_skip(self):
+    def on_skip(self) -> RunResult:
         schema_name = getattr(self.node, "schema", "")
         node_name = self.node.name
 
@@ -465,7 +465,9 @@ def on_skip(self):
                         relation=node_name,
                         index=self.node_index,
                         total=self.num_nodes,
-                        status=self.skip_cause.status,
+                        status=(
+                            self.skip_cause.status if self.skip_cause is not None else "unknown"
+                        ),
                     )
                 )
                 # skip_cause here should be the run_result from the ephemeral model
@@ -499,7 +501,7 @@ def on_skip(self):
         node_result = RunResult.from_node(self.node, RunStatus.Skipped, error_message)
         return node_result
 
-    def do_skip(self, cause=None):
+    def do_skip(self, cause: Optional[RunResult] = None) -> None:
         self.skip = True
         self.skip_cause = cause