app: fix painting with force == 0

Should be a nop, but instead does the same thing as force == 0.5.

app/paint/gimpbrushcore.c

@@ -1483,6 +1483,9 @@ gimp_brush_core_get_brush_mask (GimpBrushCore            *core,
 {
   const GimpTempBuf *mask;
 
+  if (dynamic_force <= 0.0)
+    return NULL;
+
   mask = gimp_brush_core_transform_mask (core, core->brush, op);
 
   if (! mask)

app/paint/gimpheal.c

@@ -506,6 +506,9 @@ gimp_heal_motion (GimpSourceCore   *source_core,
                                              GIMP_BRUSH_HARD,
                                              force);
 
+  if (! mask_buf)
+    return;
+
   /* check that all buffers are of the same size */
   if (src_rect->width  != gegl_buffer_get_width  (paint_buffer) ||
       src_rect->height != gegl_buffer_get_height (paint_buffer))

app/paint/gimppaintoptions.c

@@ -951,7 +951,7 @@ gimp_paint_options_get_brush_mode (GimpPaintOptions *paint_options)
   dynamic_force = gimp_dynamics_is_output_enabled (dynamics,
                                                    GIMP_DYNAMICS_OUTPUT_FORCE);
 
-  if (dynamic_force || (paint_options->brush_force > 0.0))
+  if (dynamic_force || (paint_options->brush_force != 0.5))
     return GIMP_BRUSH_PRESSURE;
 
   return GIMP_BRUSH_SOFT;