haskell: fix empty flags build failures
haskell: fix empty flags build failures

Fixes a regression introduced by #47950 in 9e3ee3e causing build
failures with the error

  cabal: The file does not exist ''.

This will occur whenever the optional :flags key isn't in the options
hash passed to install_cabal_package.

Closes #49425.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

filcab/homebrew,filcab/homebrew,filcab/homebrew,filcab/homebrew