Clean up theme_newline_prompt

No need for a variable, just echo. Allow setting $theme_newline_prompt to an empty string.

fish_prompt.fish

@@ -376,14 +376,14 @@ function __bobthefish_finish_segments -S -d 'Close open prompt segments'
     if [ "$theme_newline_cursor" = 'yes' ]
         echo -ens "\n"
         set_color $fish_color_autosuggestion
-        if [ "$theme_newline_prompt" != '' ]
-            set prompt "$theme_newline_prompt"
+
+        if set -q theme_newline_prompt
+            echo -ens "$theme_newline_prompt"
         else if [ "$theme_powerline_fonts" = "no" ]
-            set prompt '> '
+            echo -ns '> '
         else
-            set prompt "$right_arrow_glyph "
+            echo -ns "$right_arrow_glyph "
         end
-        echo -ens "$prompt"
     else if [ "$theme_newline_cursor" = 'clean' ]
         echo -ens "\n"
     end