CPack: Restore support for 0-valued version components
In commit v3.12.0-rc1~136^2 (CPack: Use project version as default for
`CPACK_PACKAGE_VERSION`, 2018-04-29) we did not account for the value of
`CMAKE_PROJECT_VERSION_{MAJOR,MINOR,PATCH}` having `0`. Fix the logic
to distinguish between unprovided version components and `0` components.
While at it, add a test case covering the behavior described in the
documentation by the original commit. The number of version components
in the package name should match those provided to the `project()`
command `VERSION` option.
Fixes: #18199
8 files changed
