fix: restclient _get_resource_uri typing — None-safe urljoin (CI Lint & Type Check)

The RestClientDefaults mixin extraction (commit 8db7c54) removed the
only `# type: ignore[union-attr]` in restclient.py. That ignore was
on `self.params.get('timeout', 60)` — but its presence happened to
shape mypy's whole-file inference such that the OTHER `self.params.get('url')`
call at line 139 was not flagged. With the per-line ignore gone, mypy
finally sees the latent issue:

  - `Value of type variable "AnyStr" of "urljoin" cannot be "str | Any | None"`
  - `Incompatible return value type (got "str | Any | None", expected "str")`

Both originate from `dict.get('url')` returning `str | None` and
urljoin requiring `str`. Replace the bare get + the misplaced
union-attr ignore with a None-safe pattern:

  base = self.params.get('url') or ''
  return urljoin(str(base), resource)

mypy clean (155 files), flake8 clean (0), 288/288 unit pass.

Author: WesleyPeng

src/main/python/taf/foundation/plugins/svc/requests/restclient.py

@@ -136,9 +136,7 @@ def _set_auth(
     def _get_resource_uri(self, resource: str) -> str:
         from urllib.parse import urljoin
 
-        return urljoin(
-            self.params.get('url'),  # type: ignore[union-attr]
-            resource
-        )
+        base = self.params.get('url') or ''
+        return urljoin(str(base), resource)
 
     # _set_default_timeout inherited from RestClientDefaults mixin.