Parenthesize nullary constraint tuples using sigPrec (#17403)

We were using `appPrec`, not `sigPrec`, as the precedence when
determining whether or not to parenthesize `() :: Constraint`,
which lead to the parentheses being omitted in function contexts
like `(() :: Constraint) => String`. Easily fixed.

Fixes #17403.

compiler/iface/IfaceType.hs

@@ -1467,7 +1467,7 @@ pprSum _arity is_promoted args
 
 pprTuple :: PprPrec -> TupleSort -> PromotionFlag -> IfaceAppArgs -> SDoc
 pprTuple ctxt_prec ConstraintTuple NotPromoted IA_Nil
-  = maybeParen ctxt_prec appPrec $
+  = maybeParen ctxt_prec sigPrec $
     text "() :: Constraint"
 
 -- All promoted constructors have kind arguments

testsuite/tests/ghci/scripts/T17403.hs

+{-# LANGUAGE ConstraintKinds #-}
+{-# LANGUAGE KindSignatures #-}
+module T17403 where
+
+import Data.Kind
+
+f :: (() :: Constraint) => String
+f = "hello world"

testsuite/tests/ghci/scripts/T17403.script

+:load T17403
+:type +v f

testsuite/tests/ghci/scripts/T17403.stdout

+f :: (() :: Constraint) => String

testsuite/tests/ghci/scripts/all.T

@@ -313,3 +313,4 @@ test('T15546', normal, ghci_script, ['T15546.script'])
 test('T16876', normal, ghci_script, ['T16876.script'])
 test('T17345', normal, ghci_script, ['T17345.script'])
 test('T17384', normal, ghci_script, ['T17384.script'])
+test('T17403', normal, ghci_script, ['T17403.script'])