ui: Fix out_flush/ui_write behavior to always flush for abstract_ui

src/nvim/term.c

@@ -1822,12 +1822,9 @@ static int out_pos = 0;                 /* number of chars in out_buf */
  */
 void out_flush(void)
 {
-  if (out_pos != 0) {
-    /* set out_pos to 0 before ui_write, to avoid recursiveness */
   int len = out_pos;
   out_pos = 0;
   ui_write(out_buf, len);
-  }
 }
 
 /*

src/nvim/ui.c

@@ -95,6 +95,10 @@ void ui_write(uint8_t *s, int len)
     return;
   }
 
+  if (!len) {
+    return;
+  }
+
   char_u  *tofree = NULL;
 
   if (output_conv.vc_type != CONV_NONE) {