Return Maybe PkgConfigDb rather than PkgConfigDb

cabal-install-solver/src/Distribution/Solver/Modular.hs

...-install-solver/src/Distribution/Solver/Modular/Solver.hs

...nstall-solver/src/Distribution/Solver/Modular/Validate.hs

...olver/src/Distribution/Solver/Types/DependencyResolver.hs

...stall-solver/src/Distribution/Solver/Types/PkgConfigDb.hs

cabal-install/src/Distribution/Client/Configure.hs

cabal-install/src/Distribution/Client/Dependency.hs

cabal-install/src/Distribution/Client/Fetch.hs

cabal-install/src/Distribution/Client/Freeze.hs

cabal-install/src/Distribution/Client/Install.hs

cabal-install/src/Distribution/Client/ProjectPlanning.hs

...nstall/tests/UnitTests/Distribution/Solver/Modular/DSL.hs

...nitTests/Distribution/Solver/Modular/DSL/TestCaseUtils.hs

...tests/UnitTests/Distribution/Solver/Modular/QuickCheck.hs