Concat arguments when enabling haddock (#10783)

* Concat arguments when enabling haddock

Arguments were unioned with <> which is left-biased, therefore -haddock
swallowed all other args as indeed described in #10782.

I think the bug was introduced by 80f0a652, which makes the expression
modified in this PR to be non-empty if documentation was enabled.

Fixes #10782

* Add test for #10783 (concatenate arguments when enabling haddock)

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Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>