Fix #111490: paint radius set to 1 (shift-smoothing but brush missing)

This affected sculpt, vertex- and weightpaint.

So attempting to (temporarily) switching to the smooth/blur tools from
another tool using the "Shift" shortcut can fail if the corresponding
smooth/blur brush is not found/missing [which was the case in the report
because the brush was deleted].

In this case, brushes dont really get switched, but blender would still
try to cache the size (because the smooth/blur brush temporarily uses
the same size as the previous brush) of the smooth brush in
`StrokeCache` (see `smooth_brush_toggle_on`). Then in
`smooth_brush_toggle_off` it was assumed brushes were actually switched
and the (non-existing) size of the (missing) smooth brush was applied to
the **actual** brush.

Now restructure code a bit so in the case of a missing brush we can
early out (without affecting the **actual** brush then).

@@ -4402,0 +4400,4 @@
+   * same, see comments for eBrushVertexPaintTool & eBrushWeightPaintTool. */
+  /* If the smooth brush is missing, brush was not switched/affected in smooth_brush_toggle_on().
+   */
+  if (!BKE_paint_toolslots_brush_get(paint, SCULPT_TOOL_SMOOTH)) {