check-coverage passes when there is no coverage data to check

* **Version**: 12.0.0, read from `node_modules/c8/package.json` (`c8 --version` prints `1.0.0`, which is #607)
* **Platform**: Windows 11 (10.0.26200), 64-bit, Node v24.15.0

`check-coverage` exits 0 when there is no coverage data to check. The report for the same run says `Unknown% ( 0/0 )`, so the fact that nothing was measured is known at report time and does not reach the threshold check.

### Reproduction

Fresh directory, `npm init -y`, `npm i c8@12.0.0`.

1. A run that produces no coverage data:

```
$ npx c8 --check-coverage --lines 100 node -e "1"
----------|---------|----------|---------|---------|-------------------
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files | 0 | 0 | 0 | 0 |
----------|---------|----------|---------|---------|-------------------
exit code: 0
```

2. The same run with `--reporter=text-summary`:

```
Statements : Unknown% ( 0/0 )
Branches : Unknown% ( 0/0 )
Functions : Unknown% ( 0/0 )
Lines : Unknown% ( 0/0 )
```

3. Control, so the threshold is visibly working when there is something to measure. `lib.js` holds one function, `t.js` is `require("./lib.js")`:

```
$ npx c8 --check-coverage --lines 100 node t.js
All files | 66.66 | 100 | 0 | 66.66 |
ERROR: Coverage for lines (66.66%) does not meet global threshold (100%)
exit code: 1
```

Three more shapes of the same thing, all exit 0:

* `npx c8 --check-coverage --lines 100 --include 'src/**' node t.js`, with the same real test as the control and a glob that matches none of it. The table prints 0 in every column and the gate passes.
* `npx c8 --check-coverage --per-file --lines 100 node -e "1"`.
* `npx c8 check-coverage --temp-directory ./empty-dir --lines 100`, which prints nothing at all and exits 0.

### Where it comes from

Two independent paths reach the same result, in case that is useful:

* `checkCoverage()` in `lib/commands/check-coverage.js` does `summary[key].pct < thresholds[key]`. For an empty map, istanbul's `CoverageSummary` sets `pct: 'Unknown'` (`istanbul-lib-coverage/lib/coverage-summary.js`), and `'Unknown' < 100` is `false`, so nothing is ever below a threshold.
* With `--per-file`, `map.files()` is empty, so the `forEach` body never runs and there is nothing to fail.

### What I am not proposing

I am not saying istanbul's `Unknown` is wrong. A file with no branches should not fail a branch threshold, and that is the same mechanism doing something useful. I also have no view I would defend on which way c8 should go: fail on an empty coverage map by default, warn, put it behind a flag, or document that `check-coverage` means "no measured metric is below the threshold". Each of those changes what existing CI does, including for people who run c8 over a process that legitimately produces no JS coverage, so the choice looks like yours rather than mine. No PR from me. If you land on a direction and want it written up, I am glad to do that.

Of the shapes above, the `--include` one seems most likely to happen by accident. A glob that stops matching after a directory is renamed leaves a coverage gate that always passes, and nothing in the output says it stopped measuring.

Start with lib/commands/check-coverage.js and istanbul-lib-coverage/lib/coverage-summary.js, then run the listed empty-coverage, unmatched --include, --per-file, and empty temp-directory reproductions. Compare the global and per-file paths and inspect existing check-coverage tests if present. Done requires an agreed behavior for empty coverage data, implemented consistently across these shapes, with regression coverage.

Written by the indexing model from the issue text.