profile/systemd-osc-context: fix overriding of PROMPT_COMMAND

profile/systemd-osc-context: fix overriding of PROMPT_COMMAND

In https://github.com/systemd/systemd/issues/39114 users are reporting
that our script overrides PROMPT_COMMAND that they had. After looking
at /etc/bashrc in Fedora, I see that it only sets PROMPT_COMMAND if
[ -z "$PROMPT_COMMAND" ]. Let's adjust the script so this continues to
work.

Fixes https://github.com/systemd/systemd/issues/39114.
(This is a bit of a stretch. 39114 was originally about SecureCRT,
but that was resolved in SecureCRT. But there was a lot of dicussion
about the prompt being overriden, which this commit should fix.)