Don't crash when formatting an empty source file. (#1524)

(Strangely, yes, I found several of these in the wild on pub.)

Author: munificent

lib/src/front_end/sequence_builder.dart

@@ -53,11 +53,13 @@ class SequenceBuilder {
     // BlockPiece.
     if (_elements.isEmpty) {
       return _visitor.pieces.build(() {
-        _visitor.pieces.add(_leftBracket!);
-        if (forceSplit) {
+        if (_leftBracket case var bracket?) _visitor.pieces.add(bracket);
+
+        if (forceSplit || _leftBracket == null) {
           _visitor.pieces.add(NewlinePiece());
         }
-        _visitor.pieces.add(_rightBracket!);
+
+        if (_rightBracket case var bracket?) _visitor.pieces.add(bracket);
       });
     }
 

test/tall/top_level/whitespace.unit

@@ -1,4 +1,10 @@
 40 columns                              |
+>>> Empty compilation unit.
+
+
+
+<<<
+
 >>> Force at least one newline between directives.
 import 'a.dart';import 'b.dart';export 'c.dart';
 <<<