Fix clean bug uncovered by 5fad121

- clean was trying to read source-builds even if it didn't exist
- add test specific to this case with other clean tests

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

@@ -120,7 +120,8 @@ cleanAction CleanFlags{..} extraArgs _ = do
     -- It would be better as part of an explicit gc step (see issue #3333)
     toClean  <- Set.fromList <$> mapM canonicalizePath extraArgs
     cacheDir <- getScriptCacheDirectoryRoot
-    caches   <- listDirectory cacheDir
+    existsCD <- doesDirectoryExist cacheDir
+    caches   <- if existsCD then listDirectory cacheDir else return []
     paths    <- fmap concat . forM caches $ \cache -> do
         let locFile = cacheDir </> cache </> "scriptlocation"
         exists <- doesFileExist locFile

cabal-testsuite/PackageTests/NewBuild/CmdClean/CleanScriptWithNoScriptsBuilt/cabal.out

@@ -0,0 +1 @@
+# cabal v2-clean

cabal-testsuite/PackageTests/NewBuild/CmdClean/CleanScriptWithNoScriptsBuilt/cabal.test.hs

@@ -0,0 +1,6 @@
+import Test.Cabal.Prelude
+import System.Environment (setEnv)
+import Distribution.Client.ScriptUtils (getScriptCacheDirectory)
+
+main = cabalTest . void $ do
+    cabal' "v2-clean" ["script.hs"]

cabal-testsuite/PackageTests/NewBuild/CmdClean/CleanScriptWithNoScriptsBuilt/script.hs

@@ -0,0 +1,6 @@
+{- cabal:
+build-depends: base
+-}
+
+main :: IO ()
+main = putStrLn "Hello World"