Fix #6219: Handle empty splice in string interpolation

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1351,6 +1351,7 @@ object desugar {
           case ts: Thicket => ts.trees.tail
           case t => Nil
         } map {
+          case Block(Nil, EmptyTree) => Literal(Constant(())) // for s"... ${} ..."
           case Block(Nil, expr) => expr // important for interpolated string as patterns, see i1773.scala
           case t => t
         }

tests/run/i6219.scala

@@ -0,0 +1,6 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert(s"hello ${} world" == "hello () world")
+    assert(s"hello ${  } world" == "hello () world")
+  }
+}