Fix fatal error caused by switch cases without any statements (#473)

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -718,13 +718,23 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
     after(node.unknownAttr?.lastToken, tokens: .space)
     after(node.label.lastToken, tokens: .break(.reset, size: 0), .break(.open), .open)
 
-    // If switch/case labels were configured to be indented, insert an extra `close` break after the
-    // case body to match the `open` break above
+    // If switch/case labels were configured to be indented, insert an extra `close` break after
+    // the case body to match the `open` break above
     var afterLastTokenTokens: [Token] = [.break(.close, size: 0), .close]
     if config.indentSwitchCaseLabels {
       afterLastTokenTokens.append(.break(.close, size: 0))
     }
-    after(node.lastToken, tokens: afterLastTokenTokens)
+
+    // If the case contains statements, add the closing tokens after the last token of the case.
+    // Otherwise, add the closing tokens before the next case (or the end of the switch) to have the
+    // same effect. If instead the opening and closing tokens were omitted completely in the absence
+    // of statements, comments within the empty case would be incorrectly indented to the same level
+    // as the case label.
+    if node.label.lastToken != node.lastToken {
+      after(node.lastToken, tokens: afterLastTokenTokens)
+    } else {
+      before(node.nextToken, tokens: afterLastTokenTokens)
+    }
 
     return .visitChildren
   }

Tests/SwiftFormatPrettyPrintTests/SwitchStmtTests.swift

@@ -78,6 +78,43 @@ final class SwitchStmtTests: PrettyPrintTestCase {
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 35)
   }
 
+  func testSwitchEmptyCases() {
+    let input =
+      """
+      switch a {
+      case b:
+      default:
+        print("Not b")
+      }
+
+      switch a {
+      case b:
+        // Comment but no statements
+      default:
+        print("Not b")
+      }
+      """
+
+    let expected =
+      """
+      switch a {
+      case b:
+      default:
+        print("Not b")
+      }
+
+      switch a {
+      case b:
+        // Comment but no statements
+      default:
+        print("Not b")
+      }
+
+      """
+
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 35)
+  }
+
   func testSwitchCompoundCases() {
     let input =
       """