shared/ptyfwd: allow window title but not background color as a valid state

Previously if a PTYForward instance had the window title set but no
background color set then it would crash in an assertion as
pty_forward_ansi_process didn't require both to be present.

systemd-vmspawn could get into this state if it failed to get the
terminal tint color.

Now any method that would have called background_color_sequence now
becomes just a NOP if the background color is not set.

This allows keeping the functionality to set window titles even if the
terminal doesn't support the background coloring.

src/shared/ptyfwd.c

@@ -270,7 +270,9 @@ static int insert_newline_color_erase(PTYForward *f, size_t offset) {
         _cleanup_free_ char *s = NULL;
 
         assert(f);
-        assert(f->background_color);
+
+        if (!f->background_color)
+                return 0;
 
         /* When we see a newline (ASCII 10) then this sets the background color to the desired one, and erase the rest
          * of the line with it */
@@ -289,7 +291,9 @@ static int insert_carriage_return_color(PTYForward *f, size_t offset) {
         _cleanup_free_ char *s = NULL;
 
         assert(f);
-        assert(f->background_color);
+
+        if (!f->background_color)
+                return 0;
 
         /* When we see a carriage return (ASCII 13) this this sets only the background */