Fix handling of empty file and file containing only comments. Tests

included.

Issue reported by Tony Morris on IRC.

runtests.hs

@@ -737,6 +737,8 @@ commentPrintSuite = testGroup "Comments"
    -- https://github.com/alanz/language-javascript/issues/15
    , testCase "issue15" (testRoundTrip "x\t=1;")
 
+   , testCase "comment-only" (testRoundTrip "// comment\n\n")
+   , testCase "empty-src" (testRoundTrip "")
    ]
 
 -- ---------------------------------------------------------------------

src/Language/JavaScript/Parser/Grammar5.y

@@ -1099,6 +1099,7 @@ FunctionBody : LBrace SourceElements RBrace { (AST.NN (AST.JSBlock [$1] $2 [$3])
 
 Program :: { AST.JSNode }
 Program : SourceElementsTop Eof { (combineTop $1 $2) {- Program -}}
+        | Eof                   { fp (AST.NN (AST.JSSourceElementsTop [$1])) }
 
 -- For debugging/other entry points
 LiteralMain :: { AST.JSNode }