eval: Fix crash on missing printValue tBlackhole case

[roberth]

Motivation

Very motivated. I hate crashes.

This fixes a bug.

Context

Fixes #8119

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or incompatible change: updated release notes

Priorities

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[edolstra]

Maybe "potential" can be dropped, since in eval output this denotes an actual infinite recursion.

[roberth]

I've explained it in a comment:

        // Although we know for sure that it's going to be an infinite recursion
        // when this value is accessed _in the current context_, it's likely
        // that the user will misinterpret a simpler «infinite recursion» output
        // as a definitive statement about the value, while in fact it may be
        // a valid value after `builtins.trace` and perhaps some other steps
        // have completed.

[yorickvP]

also fixes this one:

❯ cat flake.nix 
{
  outputs = { self }: { x = builtins.trace self { }; };
}
❯ nix build .#x
Aborted (core dumped)