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

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.