ComplexQZ would try to apply a Jacobi rotation to an empty block, which triggers a warning in static analyzers, since the corresponding `Eigen::Map` object will contain a `nullptr`.
See merge request libeigen/eigen!2043
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### Description
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More ComplexQZ fixes.
Extra semicolons are triggering some warnings and errors with `-Werror`.
Moved the `Sparse` import up to the umbrella header to avoid IWYU exports.
### Reference issue
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### Additional information
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See merge request libeigen/eigen!2036
