[test] fix ngrams test by having a better non-empty terms generator

Ref #488

test/Test/Offline/Ngrams.hs

 {-# LANGUAGE QuasiQuotes      #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE ViewPatterns     #-}
+
 module Test.Offline.Ngrams (tests) where
 
 import Prelude
 
 import Control.Lens
+import Data.Char (isSpace)
+import Data.Char qualified as Char
+import Data.Map.Strict (Map)
 import Data.Map.Strict qualified as Map
 import Data.Text qualified as T
+import Data.Tree
 import Gargantext.API.Ngrams (filterNgramsNodes, buildForest, destroyForest, pruneForest)
 import Gargantext.API.Ngrams.Types
 import Gargantext.API.Ngrams.Types qualified as NT
@@ -20,15 +25,12 @@ import Gargantext.Database.Admin.Types.Hyperdata
 import Gargantext.Database.Schema.Context
 import Test.HUnit
 import Test.Hspec
+import Test.Hspec.QuickCheck (prop)
 import Test.Instances ()
 import Test.Ngrams.Query (mkMapTerm)
 import Test.QuickCheck
 import Test.QuickCheck qualified as QC
-import Data.Tree
 import Text.RawString.QQ (r)
-import Data.Char (isSpace)
-import Data.Map.Strict (Map)
-import Test.Hspec.QuickCheck (prop)
 
 
 genScientificText :: Gen T.Text
@@ -54,12 +56,12 @@ punctuation = ",.();:-"
 
 genNgramsTermNonEmpty :: Gen NgramsTermNonEmpty
 genNgramsTermNonEmpty = do
-    singleChar <- arbitrary `suchThat` (\x -> x /= ' ' && isAllowed x)
+    singleChar <- arbitrary `suchThat` isAllowed
     txt        <- filter isAllowed <$> listOf1 genScientificChar
     pure $ NgramsTermNonEmpty $ (T.pack $ singleChar : txt)
   where
     isAllowed :: Char -> Bool
-    isAllowed s = not (s `elem` punctuation) && not (s `elem` ws) && not (isSep s)
+    isAllowed s = not (Char.isSpace s) && not (s `elem` punctuation) && not (s `elem` ws) && not (isSep s)
 
 -- In order to test the behaviour of 'docNgrams' we create wrappers around 'NgramsTerm' to have two
 -- different 'Arbitrary' flavours, one that always produces non-empty 'Text' fragments, and one that