stdenv: fix backward multiple outputs conditional

This is supposed to shareDocName to a fallback value if it can't be
determined from looking at the configure script.  But the conditional
checked whether shareDocName was set, rather than if it wasn't.  This
meant that if shareDocName had been detected from a configure script,
it would be immediately overridden by the package name, and if it
couldn't be detected, shareDocName would remain unset.

This resulted in QEMU installing files like $out/share/doc/index.html,
which should of course have been in $out/share/doc/qemu/index.html.

An interesting side effect of this is that, since
9f8751528cd89d343258dd718afa56f8590917bb when this code was added, the
detected package name has never actually been used for installing
documentation, because it would always be overridden.  So this patch
will actually enable that for the first time, four years later.

Fixes: https://github.com/NixOS/nixpkgs/issues/90486

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