Validate a missing repo mapping like it was recorded by a repo rule (#31516) ### Description `RecordedRepoMapping.getValue` now reports a mapping to a non-visible repo as `null`, which is what `Label()` in a repo rule records for an apparent repo name that isn't mapped. `RepositoryMapping#get` never returns `null`, so the value was previously the apparent name. ### Motivation A repo rule that constructs a `Label` with an unmapped apparent repo name, e.g. for an optional dependency, recorded `REPO_MAPPING:,optional_missing \0` but validated it as `optional_missing`, so neither its marker file nor its remote repo contents cache entry was ever up to date and the repo was fetched again by every server and every fresh output base. Reported in https://github.com/bazelbuild/bazel/pull/31452#issuecomment-5971363627. Work towards #31456 ### Build API Changes No ### Release Notes RELNOTES: None Closes #31516 COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/bazel/pull/31516 from fmeum:rrcc-missing-repo-mapping-value d960e2fac7430725013f2955e8b35c9382785d31 PiperOrigin-RevId: 1007317635 Change-Id: Ifd41cd6145d503aa6728de0853d284e223f41f38
{Fast, Correct} - Choose two
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