r.fillnulls: avoid locale-dependent stderr parsing

[Abhi-d-gr8]

This PR fixes a locale-dependent bug in r.fillnulls when using bilinear/bicubic interpolation.

Instead of parsing English error messages from r.resamp.bspline stderr, the script now checks for NULL cells using r.univar -g and exits early when none are present.

This avoids failures in non-English locales and simplifies the control flow.

Fixes #1495.

[marisn]

Please add a test for it.
IIRC the reason of checking of stderr contents was that r.surf.bspline reported a non 0 exit status if no NULL cells were found. But non-0 exit status is considered to indicate a failure by code executing the command. Thus tests are needed to be sure that the code doesn't fail with a bizarre error when there is no error.

[Abhi-d-gr8]

Thanks for the clarification @marisn , and apologies for missing the test initially
I’m still new to contributing here.
I’ll add a regression test for the no-NULL case as suggested.

[Abhi-d-gr8]

Added a regression test (test_no_nulls) covering the no-NULL input case to ensure r.fillnulls exits successfully without relying on stderr parsing. CI is running.

[github-actions]

[ruff] _{reported by reviewdog 🐶}

Suggested change

	def test_no_nulls(self):
	def test_no_nulls(self):