Do not build code unit map when test has no coverage targets

[janedbal]

Problem

• The Test\Target\Mapper is built even when no test in the entire suite uses #[CoversClass] / #[UsesClass].
• Building the mapper means MapBuilder::build() statically analyses every file in the filter, so a suite that requests any coverage report pays a full source-tree analysis per PHP process, even though the resulting map is never consumed.
• PHPUnit passes an empty TargetCollection to CodeCoverage::validate() / append() for every test that declares no coverage targets, and both call sites build the mapper before noticing the collection is empty.

Real-world numbers (app with ~24k files under <source>, 5 fast unit tests, --coverage-cobertura, PCOV, cold cache):

	wall time	peak memory
latest 12.5.7	53.5 s	657 MB
with this change	10.6 s	426 MB

Without this patch, we are unable to migrate from PHPUnit 11 to 12+ as our overall CI time for tests inscreases from ~7 mins to ~12 mins.

Notes:

• Generated report is byte-identical.
• Projects that do use #[CoversClass]/#[UsesClass] are unaffected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (4b61c53) to head (3b3a16d).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               12.5    #1244      +/-   ##
============================================
- Coverage     89.39%   89.31%   -0.09%     
- Complexity     1404     1407       +3     
============================================
  Files           105      105              
  Lines          4724     4726       +2     
============================================
- Hits           4223     4221       -2     
- Misses          501      505       +4     

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[sebastianbergmann]

Merged manually, thank you.

[janedbal]

Ref, thank you!