fix(tvix/eval): disallow empty but allocated string contexts r/8167

Both `Some(NixContext::new())` and `None` represent empty contexts,
but the former trips up `NixString::has_context`, and seems likely
to trip up other things.

We could hide the difference in the accessors, but we don't really
*want* the distinction to exist, since heap-allocating a null value
is pretty much always a mistake.

Change-Id: Ie84d26fb0d4b59e68354891ba13bde3bae40ab6e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11712
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

0 files changed, 0 insertions, 0 deletions