ast: handle boxing of erroneous and empty values gracefully

[maxteufel]

Fixes #1765.

[michaellilltokiwa]

This is another approach of fixing the bug, i.e. overriding the tagging for the used error value:

diff --git a/src/dev/flang/ast/Expr.java b/src/dev/flang/ast/Expr.java
index 5b53c73e..5427a9c4 100644
--- a/src/dev/flang/ast/Expr.java
+++ b/src/dev/flang/ast/Expr.java
@@ -602,6 +602,11 @@ public abstract class Expr extends ANY implements Stmnt
     NO_VALUE = new Call(SourcePosition.builtIn, Errors.ERROR_STRING)
     {
       { _type = Types.t_ERROR; }
+      @Override
+      Expr box(AbstractType frmlT)
+      {
+        return this;
+      }
     };
   }