Fix edge case for block diagonalization

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (2321de2) 84.45% compared to head (1861eb7) 85.10%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #323 +/- ## ========================================== + Coverage 84.45% 85.10% +0.64% ========================================== Files 41 41 Lines 2200 2222 +22 ========================================== + Hits 1858 1891 +33 + Misses 342 331 -11 ``` | [Files](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/323?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev) | Coverage Δ | | |---|---|---| | [src/Certificate/Symmetry/block\_diag.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/323?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL0NlcnRpZmljYXRlL1N5bW1ldHJ5L2Jsb2NrX2RpYWcuamw=) | `98.88% <98.66%> (+7.16%)` | :arrow_up: | | [src/Certificate/Symmetry/wedderburn.jl](https://app.codecov.io/gh/jump-dev/SumOfSquares.jl/pull/323?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=jump-dev#diff-c3JjL0NlcnRpZmljYXRlL1N5bW1ldHJ5L3dlZGRlcmJ1cm4uamw=) | `0.00% <0.00%> (ø)` | |

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