Parser: allow empty multi-line deprecation warnings

This should work,

    {-# DEPRECATED someFunction [] #-}

Test Plan: parser/should_compile/T3303

Reviewers: bgamari, austin

Reviewed By: austin

Subscribers: mpickering

Differential Revision: https://phabricator.haskell.org/D1433

GHC Trac Issues: #11044

compiler/parser/Parser.y

@@ -1422,6 +1422,7 @@ stringlist :: { Located (OrdList (Located StringLiteral)) }
                                return (sLL $1 $> (unLoc $1 `snocOL`
                                                   (L (gl $3) (getStringLiteral $3)))) }
     | STRING                { sLL $1 $> (unitOL (L (gl $1) (getStringLiteral $1))) }
+    | {- empty -}           { noLoc nilOL }
 
 -----------------------------------------------------------------------------
 -- Annotations

testsuite/tests/parser/should_compile/T3303.hs

@@ -6,3 +6,5 @@ import T3303A
 bar :: Int
 bar = foo
 
+bar2 :: Int
+bar2 = foo2

testsuite/tests/parser/should_compile/T3303.stderr

 
-T3303.hs:7:7: Warning:
+T3303.hs:7:7: warning:
     In the use of ‘foo’ (imported from T3303A):
     Deprecated: "This is a multi-line
                  deprecation message
                  for foo"
+
+T3303.hs:10:8: warning:
+    In the use of ‘foo2’ (imported from T3303A): Deprecated: ""

testsuite/tests/parser/should_compile/T3303A.hs

@@ -8,3 +8,8 @@ module T3303A where
 foo :: Int
 foo = 4
 
+-- Empty list should work too (#11044).
+{-# DEPRECATED foo2 [] #-}
+
+foo2 :: Int
+foo2 = 4