JVM: Add support for call target type being unit (Java's void), fix #…

…1902

The code to filter out unit type values was present for the arguments already,
we need this as well for the target that is passed as a first argument and that
may be unit type as well.

src/dev/flang/be/jvm/Types.java

@@ -533,7 +533,11 @@ String descriptor(boolean explicitOuter, int cl, boolean pre)
       {
         var or = _fuir.clazzOuterRef(cl);
         var ot = _fuir.clazzResultClazz(or);
-        as.append(resultType(ot).descriptor());
+        var at = resultType(ot);
+        if (at != PrimitiveType.type_void)
+          {
+            as.append(at.descriptor());
+          }
       }
     for (var ai = 0; ai < _fuir.clazzArgCount(cl); ai++)
       {