[gimp] Revert "app: do not add undo step when clearing an already empty channel."

[Michael Natterer <mitch src gnome org>]

commit 19a28cc7091f9ed3eef9a5da123e1a5e000df538
Author: Michael Natterer <mitch gimp org>
Date:   Sun Nov 12 23:42:43 2017 +0100

    Revert "app: do not add undo step when clearing an already empty channel."
    
    This reverts commit 9c940722131678fd7b4246b100d5f1430f5f327c.
    
    Such things do not belong into the core but into the GUI.

 app/core/gimpchannel.c |   10 +++-------
 1 files changed, 3 insertions(+), 7 deletions(-)
---
diff --git a/app/core/gimpchannel.c b/app/core/gimpchannel.c
index 4f3b063..964a393 100644
--- a/app/core/gimpchannel.c
+++ b/app/core/gimpchannel.c
@@ -1317,14 +1317,10 @@ gimp_channel_real_clear (GimpChannel *channel,
 {
   if (push_undo)
     {
-      if (! channel->empty)
-        {
-          /* Don't push an undo if the channel was already empty. */
-          if (! undo_desc)
-            undo_desc = GIMP_CHANNEL_GET_CLASS (channel)->clear_desc;
+      if (! undo_desc)
+        undo_desc = GIMP_CHANNEL_GET_CLASS (channel)->clear_desc;
 
-          gimp_channel_push_undo (channel, undo_desc);
-        }
+      gimp_channel_push_undo (channel, undo_desc);
     }
   else
     {