core: add flag "input_get_empty" in buffer

The default value is 0 (legacy behavior).
When it is set to 1, an empty input (just by pressing Return with nothing in
input) is sent to the input callback, which receives an empty string.

src/core/wee-input.c

@@ -194,8 +194,7 @@ input_data (struct t_gui_buffer *buffer, const char *data)
 
     rc = WEECHAT_RC_OK;
 
-    if (!buffer || !gui_buffer_valid (buffer)
-        || !data || !data[0] || (data[0] == '\r') || (data[0] == '\n'))
+    if (!buffer || !gui_buffer_valid (buffer) || !data)
     {
         return WEECHAT_RC_ERROR;
     }
@@ -213,12 +212,12 @@ input_data (struct t_gui_buffer *buffer, const char *data)
                                    data);
 
     /* data was dropped? */
-    if (new_data && !new_data[0])
+    if (data[0] && new_data && !new_data[0])
         goto end;
 
     first_command = 1;
     ptr_data = (new_data) ? new_data : data;
-    while (ptr_data && ptr_data[0])
+    while (ptr_data)
     {
         /*
          * if the buffer pointer is not valid any more (or if it's another

src/core/wee-upgrade.c

@@ -509,6 +509,9 @@ upgrade_weechat_read_buffer (struct t_infolist *infolist)
     ptr_buffer->input = infolist_integer (infolist, "input");
     ptr_buffer->input_get_unknown_commands =
         infolist_integer (infolist, "input_get_unknown_commands");
+    ptr_buffer->input_get_empty =
+        infolist_integer (infolist, "input_get_empty");
+
     if (infolist_integer (infolist, "input_buffer_alloc") > 0)
     {
         ptr_buffer->input_buffer =