Fix #149438: Set Uniform Opacity removes Opacity from other stokes

In case there is no "fill_opacity" attribute already, this was added,
but not initialized with 1.0 for unselected strokes.
This is now fixed (similar to 089a435376).

Also added a `finish()` call that was missed in 9759a9df5e it seems.

Pull Request: https://projects.blender.org/blender/blender/pulls/149444

source/blender/editors/grease_pencil/intern/grease_pencil_edit.cc

@@ -1072,11 +1072,14 @@ static wmOperatorStatus grease_pencil_set_uniform_opacity_exec(bContext *C, wmOp
     bke::curves::fill_points<float>(points_by_curve, strokes, opacity_stroke, opacities);
 
     if (SpanAttributeWriter<float> fill_opacities = attributes.lookup_or_add_for_write_span<float>(
-            "fill_opacity", AttrDomain::Curve))
+            "fill_opacity",
+            AttrDomain::Curve,
+            bke::AttributeInitVArray(VArray<float>::from_single(1.0f, curves.curves_num()))))
     {
       strokes.foreach_index(GrainSize(2048), [&](const int64_t curve) {
         fill_opacities.span[curve] = opacity_fill;
       });
+      fill_opacities.finish();
     }
 
     changed = true;